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The Engineering Basics of CTC
By Michael J. Burgett

Part 1 The Basics
Part 2 Clearing a Signal at an Interlocking
Part 3 Intermediate Signals
Part 4 Signal Aspects and Indications
Part 5 The Control Machine

Part 1 "The Basics"

This information is intended to give one a better understanding of how Centralized Traffic Control (CTC) operates. All information is of a general description and does NOT focus on one particular prototype, but instead on standard practices used throughout the railroad industry. CTC and signaling in general is such an involved subject, that I will try keeping simple, yet I do not want to leave out any key points. Hope you enjoy this information and happy model railroading!

An early signal engineer once said, "The art of signaling may be quite truly termed the art of saving seconds safely." You could say that the railroad industry took this idea and ran with it purchasing CTC for much of their busy mainlines. Beginning in the mid-30's the railroads continued construction from one division to another as time and money became available. The priorities were established according to traffic density and savings. On average CTC saved trains 42 seconds per mile and permitted passenger trains to make up lost time without delaying other trains. Compared to timetable and train order operations this was a vast improvement, not to mention CTC greatly reduced the danger of human error. The economics alone of eliminating telegraph offices and interlocking operators made it justifiable to install CTC.

CTC is defined as "a block system under which train movements are authorized by block signals whose indications supersede the superiority of trains for both opposing and following movements on the same track." CTC provides centralized control for signals and switches within a territory of defined limits, controlled from a single control console known as a control machine. Because the distance between the control machine and the field locations in CTC territory usually extends a number of miles, the system is controlled by code and carrier equipment. A direct-wired arrangement would require one pair of wires for each controlled signal and switch within the CTC territory, and would not be economical. The code and carrier system allows the CTC system to operate over only two line wires, known as the code line. This circuit is the most important pair of wires on the pole line. More so than the dispatcher's phone circuit and is the first to be repaired if major damage to the pole line occurs. The control machine provides a means for the dispatcher to monitor wayside conditions and to initiate the control codes thru push buttons and levers. The control codes are sent to the field equipment via the code line to control the corresponding signals and switches.


Each end of a siding controlled by the dispatcher is called an interlocking. In contemporary railroading this is called a control point (CP). The definition of an interlocking is: "An arrangement of signals and signal appliances so interconnected that their movement must succeed each other in proper sequence and for which interlocking rules apply." Each signal governing entrance to an interlocking is called an interlocking signal or home signal. These signals display stop (rule 292) as their most restrictive indication.

When a less restrictive indication than "stop" is displayed, a signal is said to be "cleared." The track between the outer opposing signals is referred to as the interlocking limits. The detection circuit (track circuit) within the interlocking limits is known as the detector track circuit. The dispatcher calls this same circuit the O.S. section, O.S. meaning "on sheet." When this circuit becomes occupied by a train the dispatcher will show on his train sheet that this train is by the location that corresponds with that particular circuit.

Not all signals in CTC territory are controlled by the dispatcher. Intermediate signals (signals with number plates) generally govern movements between interlockings and are commonly spaced at one train length apart. These signals are usually operated automatically within the CTC system and their aspect determined by train location and track condition. Signals controlled by the dispatcher do not have number plates and are two or three unit signals. Each signal unit is commonly referred to as an arm. This comes from the early days when the blade on a semaphore signal was called an arm. Each signal unit may display one, two or three colors. Some people call each color in a signal unit an aspect; do not be confused: an aspect is the appearance of a signal conveying an indication either by one color being displayed or by a combination of colors being displayed. Each color should be referred to as a light. Signal units that display only one color are called marker lights.

Signals in CTC territory controlled by the dispatcher have the prefix on the control machine L or R. This comes from the days when the operators were located in cabins along the tracks. As you might imagine L signals were to his left and R signals were to his right. When CTC began to be installed, L signals became westbound and R signals became eastbound. Most railroads list the tracks in their timetables as east and west, even though they may geographically run north and south. In the event that the tracks do run north and south, north becomes west and south becomes east. In conjunction with the L or R prefix signals are numbered evenly, 2, 4, 6, 8, etc. For example, a westbound signal at the east end of the siding could be labeled L88. The eastbound signal would be R88. Switches and electric locks are labeled with odd numbers, 1, 3, 5, 7, etc. so the switch at the east end of the siding would be 87. The numbering sequence usually started at a low number at the west end of the territory and would get higher as you moved east, however the numbering sequence did not continue consecutively. As shown in Figure 1 the west end of the siding is signal L and R82 while the east end is L and R88. The reason for this is to allow for expansion. For example, if we wanted to add a crossover in the middle of the siding between the east and west ends we are able to use L and R84 and switch 83 or L and R86 and switch 85.

Power switches operated by either electric or air are labeled on the control machine N and R for normal and reverse, normal being the main track route and reverse being the diverging route, either from one main track to another or from the main track to a siding.

Electrically-locked switches are also labeled N and R for normal and reverse. Normal being locked for the main track and reverse being unlocked and the switch lined for the siding. An electric lock provides a means of locking a manually operated switch with the signal system so that the switch cannot be operated unless traffic conditions permit.


Figure 1 shows a standard layout of signals at both the east and west ends of an interlocked siding in CTC territory. The dispatcher controls all signals and switches at this type of location.


Figure 2 shows the standard arrangement of signal units for this type of location. Home signal R174 governs eastward movements over the power switch in the facing point direction either across the main track or into the siding. This signal is known as the facing-point signal. The top or (A) unit has three-colors; green, yellow, and red. This unit governs eastward main track movements through the interlocking and to the next governing signal. The lower or (B) unit has two-colors; yellow and red. This unit governs eastward movements through the interlocking from the main track into the siding. Home signals L174 and LB174 governs westward movements over the power switch in the trailing point direction. Both signals are referred to as the trailing point signals. Both signals govern westward movements through the interlocking and to the next governing signal. Signal L174 governs this movement across the main track. The (A) unit of this signal is again a three-color unit; green, yellow, and red and like R174 it also governs main track movements, but in the westward direction. Signal LB174 governs the trailing point movement from the siding to the main track. This signal is of the dwarf type. A dwarf signal is a low signal used in yards or at a siding or where clearance restricts the use of a high signal. This signal also has three colors; green, yellow, and red. The other end of the siding would have the identical arrangement of signal units, but using the next couple of numbers higher in the sequence. The prefix L and R is also reversed to show the proper direction at the East end.

Part 2

"Clearing a Signal
at an Interlocking"



Let's now look at the requirements for clearing a signal for any given route through an interlocking. I will list each condition that the signal system will check. This should help in setting up your own signals to operate more like the prototype. Looking at Figure 1, let's assume that there is a westbound train approaching signal L88 at the east end of the siding. The dispatcher has decided that this siding will be the best place for the westbound to meet an eastbound. The dispatcher from his control will send out the control code to reverse switch 87. Before switch 87 will throw the signal system will check three conditions:

1. No train is within the interlocking limits (occupying the detector track circuit) - this protects against throwing the switch under a train.
2. No home signals at this interlocking are cleared for a route over the switch.
3. Against throwing for a predetermined time delay, after a home signal governing movement over the switch is cleared, and then put back to stop by the dispatcher.

Condition #3 is known as "time locking". Time locking protects against the dispatcher clearing a signal for an approaching train, then putting the signal back to stop and changing the route. When the dispatcher manually puts the signal to stop, the signal system will make him wait a set time before he can change the route or clear the opposing home signals. This time interval is to allow the approaching train sufficient time to stop safely before his original route is changed. The formula that we use on the railroad to determine this time interval takes into account track speed, curvature, and other pertinent train handling factors. It can be as short as two minutes and as long as eight minutes. Considering that most modelers run their railroads using a fast clock, running eight or even two minutes of time for time locking would not be practical. My experience has been a delay of one minute is sufficient and seems to work quite nicely. You may find for your railroad you want a longer or shorter time interval, or you may not want to emulate time locking at all. If any of the above three requirements are not met, the switch is locked and cannot be thrown under power:

After switch 87 has thrown to the reverse position it will indicate to the control machine that it has responded and is now in the reverse position. Next the dispatcher will send out the control code to clear signal L88 for the westbound to enter the siding. Again the signal system will check several conditions.

1. No train is occupying the detector track circuit or the block protecting the siding. This block extends between signal LC82 and RC88.
2. That home signal RC88 is not cleared for an eastward movement and that this Signal is not in the process of running time for time locking.
3. Note that this condition is only checked if switch 81 is in the reverse position. No train is occupying the detector track circuit at the west end of the siding and that signal R82 is not cleared or running time.
4. Switch 87 is in the reverse position.

If all these conditions are met signal L88 will display a red over yellow aspect and the field equipment will indicate to the dispatcher that this signal is now cleared. The red over yellow aspect is a restricting indication (Rule 290). This signal informs the engineer that he may proceed not exceeding 15 MPH and prepared to stop short of an obstruction, or anything else that may require him to stop. Aspects and indications will be discussed more thoroughly in part four of this series.

Now the dispatcher must clear the appropriate signal for the eastbound train. Assuming switch 81 at the west end of the siding is in the normal position, the dispatcher will send out the control code to clear signal R82. The signal system will check the same conditions that were previously checked to clear signal L88 but, for the route signal R82 will be governing (i.e. down the main track):

1. No train is occupying the detector track circuit at this interlocking and the block between signal L82 and R88 is not occupied.
2. Signal L82 is not cleared for a westbound movement or in the process of running time.
3. Switch 81 is in the normal position.

Considering that switch 87 at the east end of the siding is already in the reverse position to allow the westbound to enter the siding the signal system knowing this will not check signal L88 or the detector track circuit at the east end. Once these conditions have been checked signal R82 will display a yellow over red aspect. The yellow over red aspect is an approach indication (Rule 285) and informs the engineer that he may proceed not exceeding 30 MPH and to be prepared to stop at the next signal (i.e. signal R88). The dispatcher has now successfully set up a meet between two trains. All of this took him less than one minute to do. This is what makes CTC so efficient. The dispatcher has no train orders to write and relay to a telegraph operator, who then has to hoop them up to the appropriate train. Also each train would then have to stop and throw the appropriate switches by hand.

After the westbound train has passed the "cleared" signal (i.e. signal L88), it moves into the interlocking limits, it is now occupying the detector track circuit. The signal now automatically returns to stop, and the field equipment will indicate to the control machine that the detector track circuit is occupied and that the home signal is at stop.

Once the last car has cleared the interlocking limits and is no longer occupying the detector track circuit, the dispatcher will send out the control code for switch 87 to throw to the normal position. After the switch has thrown to the normal position and indicated to the control machine that it has responded, the dispatcher will clear signal R88 for the eastbound train to proceed. The signal system will check several conditions to insure that the track conditions are safe for the eastbound to proceed:



1. No train is occupying the detector track circuit or the block that extends between signal L88 and the next governing signal.
2. Signal L88 is not in the process of running time.
3. Switch 87 is in the normal position.
4. All westbound intermediate signals between the east end of this siding and the west end of the next siding are in the red position and that no westbound signals are cleared or running time at the west end of the next siding.

We will discuss in part three intermediate signals and the conditions required to clear a signal into single track between sidings. If all four of the above conditions are met signal R88 will display a green over red aspect. The green over red aspect is a clear indication (Rule 281) and informs the engineer that he may proceed at authorized track speed.

Whether you're modeling a single track line with passing sidings of some western railroad or a massive interlocking in Chicago, the general principle is the same. The signal system will always make the appropriate checks to see if the route that the dispatcher and/or operator has requested is indeed safe for train movement.


Part 3 Intermediate Signals

In this part we look at the operation of the signal system as applied to movements between interlockings and how each individual signal will react to different conditions established by the dispatcher.

Generally speaking, signals that govern movements between interlockings are not controlled by the dispatcher. Instead they operate automatically within the CTC system and their aspect determined by train location and track condition. These signals are known as intermediate or automatic signals. These signals can be located on a mast, bridge or cantilever structure and in some rare cases as a dwarf signal. Intermediate signals are defined as "A block signal which conveys Stop and Proceed (Rule 291) as its most restrictive indication." This indication (Stop and Proceed Rule 291) informs the engineer that he must stop at the red intermediate signal and then he may proceed not exceeding 15 MPH and prepared to stop short of an obstruction, or anything else that may require him to stop. Some railroads changed the old "Stop and Proceed Rule 291" to a Restricted Proceed indication. This new indication was intended to save fuel, studies showed that it took three GP-9's 40 or more gallons of fuel to accelerate a 12,000 ton train to 40MPH. With this "new indication" trains were not required to stop but instead allowed to simply proceed by the red intermediate signal at restricted speed.

Intermediate signals can be identified by the presence of a number plate which is attached to the mast or in adjacent proximity to the signal. The number assigned to each intermediate signal reflects the subdivision's mile post in tenths of a mile. In keeping with most railroads practice of numbering eastbound traffic even and westbound traffic odd, the eastward signals take the even mile post number and the westward signals take the odd mile post number to the nearest tenth of a mile.


Figure 4 shows a typical layout of intermediate signals between interlockings. Intermediate signals are commonly spaced at one train length apart or approximately two miles. The number of intermediate signals between interlockings is determined by the distance between these locations, the greater the distance the more intermediate signal locations required. This allows for greater flexibility in the operation of the signal systems. For example if the dispatcher has a fleet of westbound trains to run these trains can follow one another at a safe distance apart between interlockings. In this arrangement undue delays to trains are eliminated by allowing following movements, while safety is still maintained by preventing opposing movements between adjacent sidings.

In figure 4 lets assume that all conditions are normal (i.e. no trains are in this territory and no home signals are cleared for a route at interlocking A or B). In this condition all home signals will display Stop (Rule 292), signals 1203 (MP 120.3), 1204 (MP 120.4), 1227 (MP 122.7), and 1228 (MP 122.8) will be dark (i.e. not lit). These signals are normally dark and will light only if a train is occupying the block immediately ahead of the signal. This feature is known as Approach Lighting. Even though these signals are dark the associated circuitry will be as such that if they where lit they would display the following. Signal 1203 and signal 1228 will display an Approach indication (yellow Rule 285). Additionally signals 1204 and 1227 will display a Clear indication (green Rule 281).


Figure #5 shows an intermediate signal location in double track territory. These signals operate much like those in figure 4. A common practice is to light all intermediate signals at this location together for both tracks under the same conditions, regardless of what track is occupied. This is known as "two track approach lighting". It is important to note that some railroads (C&O, GTW, DT&I, etc.) did not approach light home signals, but instead lit them continually so that their aspects could be seen by maintenance of way employees, giving them advance warning of approaching trains.

The dispatcher will now clear a westward signal ( i.e. signal L14) at interlocking (B) by positioning signal lever number 14 to the L position and pushing the code start button. Signals 1228 and 1204 will now automatically go to red. This ensures that a train does not receive a favorable indication into a block if an opposing route is established. This is called a tumble down, all opposing intermediate signal will tumble to red all the way back to the next interlocking, like a set of dominos. This feature keeps the dispatcher from clearing an eastward signal at interlocking (A) and prevents him from making a "corn field meet." Additionally all of the intermediate signals between these two interlockings will now light. Once the tumble down is complete signal L14 will display a Clear indication (green over red Rule 281). The dispatcher will now clear signal L6 for the westbound to enter the siding at interlocking (A), after all of the conditions discussed in part two of this series are met signal L6 will display an Restricting indication (red over yellow Rule 290). Signal 1203 will not upgrade in this situation to a Clear indication (green) like one might imagine but instead will remain an Approach indication (yellow). This is because a Restricting signal may require that the engineer stop his train for an obstruction shortly beyond the home signal.

As the westbound train proceeds by each signal it will automatically turn to red. To improve traffic flow, it is desirable to permit trains to follow each other in the same direction at a safe distance apart. To accomplish this the signal system will electrically set a traffic stick. This "traffic stick" will enable the dispatcher to clear signal L14 once again and allow a second westbound to follow one block behind. Signal L14 will now display an Approach indication (yellow over red Rule 285.) This feature also keeps the opposing intermediate signals at red behind the westbound train until the block is clear. Each intermediate location will set this traffic stick and enable the proceeding signal to display an Approach indication. Once the westbound train has progressed and the caboose is no longer occupying the detector track circuit of interlocking (A) all intermediate signals will now return to their normal status. The eastward signals work exactly the same for the eastward direction.

Part 4 Signal Aspects and Indications

Aspects and Indications are the basic building blocks of a signal system. Aspects are the means in which the signal conveys its indication to the train crew, either by one color being displayed or by a combination of colors being displayed. The Indication is the action that the aspect uses to inform the train crew to take, thereby insuring safe operation of their train. These indications give information as to the allowable speed of interlocked turnouts, block conditions, and information as to the next signal to be encountered.

Early on, the Association of American Railroads (AAR), now AREMA or American Railway Engineering and Maintenance-of-way Association, set guidelines for standard use of signal aspects and indications; however, this did not prevent each railroad from creating its own standards for unique situations. Despite the AREMA "standard", railroad signal aspects soon became as unique as each railroad's paint schemes, logo, and structures. For modelers this creates quite a challenge. Not only does one model a particular railroad's motive power and rolling stock, but to model its unique operating practices. Modeling operating practices might include modeling a commonly used locomotive consist, the use of two cabooses on a local freight, and of course the railroad's signal aspects.

To understand why railway signal aspects go beyond using the simplest two color system of red and green, we must first understand some important train handling factors. A loaded coal train of any where between 90 to 150 cars can weigh 20,000 to 25,000 tons. Depending on grade and track curvature the distances to stop or reduce the speed safely of such trains can vary greatly.

This distance to stop or reduce the speed is referred to as "braking distance", see figure 6B for standard braking distances. This also explains why some subdivisions have tonnage speed restrictions. These speed restrictions (listed in the timetable) insure that a high tonnage train does not exceed a speed that will prevent stopping within the required distances.

Keeping braking distances in mind, the train crew must have plenty of advance warning of any conditions that will affect the movement of their train. These conditions may be anything from an upcoming stop signal to a switch that the dispatcher has set to crossover from one main track to another or to take a siding. The upcoming turnout may be of many different sizes depending upon the requirements of the physical track layout, typical use, and the philosophy of the chief engineering officer. Locations where turnouts are employed to cross trains over from one main track to another, or to enter single from double track will typically have larger turnouts capable of higher speed movements than those employed to enter a siding or a branch line. The railroad's objective is to balance cost with efficiency, just as we try to accomplish on our model railroads.

One similarity between model railroading and the prototype is that they both use a numeric system to indicate the size of the turnout. (The NMRA and the AAR established these numbering systems respectively.) Although the prototype turnout numbering system is not the same as the model, the basic principle is. The turnout number is derived from the angle of the turnouts frog, this measurement is referred to as the "frog angle". The turnout frogs used in modeling are the same as the prototype, and the length of lead and radius of curvature are comparable to scaled-down measurements. In this system the larger the number the less severe the angle of the frog, therefore simple physics tell us that the larger frog angles will accommodate higher speed movements through them on the diverging route than those with smaller frog angles.




Figure 6 shows a chart that I derived for use on my own model railroad. I based this chart on information in the AREMA track standards manual.

With this information one can quickly see why there are so many different signal aspects needed. Not only does the signal need to inform the train crew of approaching block conditions and of the next signal to be encountered, but the speed permitted through a given turnout when set to take the diverging route. To make identification of the signal aspects easier and quicker for the train crew they are given names that corresponds with the action that must be taken. In conjunction with these names, the speed at which the signal is authorizing the train movement is also named.


See figure 7 for a listing of speed classes with their associated names.

The following shows some standard signal aspects and indications used on many North American railroads. We will now look at each signal aspect and indication and how and where it would most commonly be used:

CLEAR (RULE 281)

This indication undoubtedly is the engineer's favorite as it denotes that he has a minimum of two unoccupied blocks ahead of him and all switches within the route govern by the Clear signal are aligned for a non diverging move. With these conditions established, the clear indication allows him to operate at maximum authorized speed. This is sometimes referred to by railroad employees as "track speed". Maximum authorized speed varies depending on geography track structure. It can vary any where from 40 MPH in mountainous terrain to 60 MPH on level and tangent track. The maximum authorized speed for any given location on a given subdivision is located in the timetable.

APPROACH MEDIUM (RULE 282)

Approach Medium is used several different ways, the most common way it is used, is to inform the engineer that the next signal to be encountered will restrict his movement to medium speed (30 MPH). The next signal will then require the engineer to have the train at a speed not to exceed medium speed - usually a Medium Clear (Rule 283), Medium Approach Medium (Rule 283-A) or a Medium Approach (Rule 286) indication. In this arrangement the engineer has the distance from the Approach Medium indication to the Medium Clear, Medium Approach Medium or Medium Approach indication (i.e. one block length) to reduce the speed from maximum authorized speed to the required medium speed.

Another way the railroad uses the Approach Medium indication is to reduce train speed two blocks in advance of a stop indication (Rule 292). This is used where there is not sufficient braking distance between signals to safely stop the train.

In figure 9 the maximum authorized speed is 40 MPH with a 0.8% descending grade. This requires a braking distance of 1667 feet. Considering that the distance from signal 72R to 78R is only 1287 feet, 380 feet short of the required braking distance, the Approach Medium indication is then employed on intermediate signal 3598. In this application, an approaching eastbound train will start reducing its speed upon receiving the Approach Medium indication. The engineer must now have their train reduced to medium speed at or before reaching the next signal (i.e. signal 72R). Using the exact same mathematical formula as before with the speed reduced from 40 MPH (maximum speed) to 30 MPH (medium speed), the required braking distance is now only 938 feet. This is ample distance for the eastbound train to stop safely at signal 78R.

In a model railroad application, I like to employ the Approach Medium indication to simulate short braking distance if the distance between a particular set of signals is less then one average train length apart. This works quite nicely in the case of a short block where an operator is more likely to miss an Approach indication and run by the next signal displaying "Stop" into the back end of a brass caboose. As we operate our model train across our railroad and encounter multiple "Clear" indications we tend to become complacent and distracted just as the prototype engineers can. This is not surprising considering that one of the most enjoyable things about this hobby is to socialize with our fellow modelers. It is important to note that some railroads would use an Advance Approach Rule 285A (flashing yellow) in lieu of the Approach Medium aspect.

APPROACH LIMITED (RULE 281B)

Approach Limited is used to inform the engineer that the next signal to be encountered will restrict his movement to medium speed (40 MPH). The next signal will then require the engineer to have the train at a speed not to exceed limited speed - usually a Limited Clear (Rule 283A). In this arrangement the engineer has the distance from the Approach Limited indication to the Limited Clear indication (i.e. one block length) to reduce the speed from maximum authorized speed to the required limited speed.

MEDIUM CLEAR (RULE 283)

The Medium Clear indication is used to communicate to the engineer that the dispatcher has set a power operated turnout for the diverging route and that the speed on that route can not exceed medium speed (30MPH). It also indicates a minimum of two unoccupied blocks are ahead of him. The "diverging route" may be to crossover from one main track to another, enters a siding and/or enters double from single track.

Figure 10 show one common use for the Medium Clear indication. In this illustration the dispatcher has set crossover 9 to the reverse position to cross an eastbound over from track 1 to track 2 at interlocking (B). Signal 10R is displaying the Medium Clear indication in addition signal 2R at interlocking (A) is displaying the Approach Medium indication to advise the eastbound engineer of the upcoming medium speed restriction. Once the last car of the eastbound train clears interlocking (B) the engineer may then resume maximum authorized speed.

LIMITED CLEAR (RULE 281C)

The Limited Clear indication is used to communicate to the engineer that the dispatcher has set a power operated turnout for the diverging route and that the speed on that route can not exceed Limited speed (40MPH). It also indicates a minimum of two unoccupied blocks are ahead of him. The "diverging route" may be to crossover from one main track to another, or to enter double from single track.

MEDIUM APPROACH MEDIUM (283-A)

The Medium Approach Medium indication is used to indicate to the engineer that the dispatcher has set a power operated turnout for the diverging route and that the speed on such route can not exceed medium speed (30 MPH). It also informs them that they must not exceed medium speed approaching the next signal.

Figure 11 show one common use for the Medium Approach Medium indication. In this illustration the dispatcher has set crossover 9 to the reverse position to cross an westbound over from track 2 to track 1 at interlocking (B) and back over to track 2 at interlocking (A). Signal 12L is displaying the Medium Approach Medium indication. The approaching westbound engineer will proceed through interlocking (B) and over to track 1 not exceeding medium speed. He will then proceed not exceed slow speed approaching the next signal (i.e. signal 2L). The next signal "2L" at interlocking (A) is displaying a Medium Clear indication (Rule 283) to cross the westbound back over to track 2. Our westbound train approaching interlocking (A) is already restricted to medium speed by the previous signal indication received at signal 12L (a Medium Approach Medium indication). This is ample speed to negotiate the medium speed turnout back over to track 2 as indicated by the Medium Clear indication on signal 2L.

APPROACH SLOW (RULE 284)

The Approach Slow indication is used to inform the engineer that the next signal to be encountered will restrict their movement to slow speed (15MPH). The next signal will then require them to have the train at a speed not to exceed slow speed, usually a Slow Clear (Rule 287) or Slow Approach (Rule 288) indication. In this arrangement the engineer has the distance from the Approach Slow indication to the Slow Clear or Slow Approach indication (i.e. one block length) to reduce the speed from maximum authorized speed to the required slow speed.

APPROACH (RULE 285)

This indication denotes that the engineer has only one unoccupied block ahead of them and all switches within the route govern by the Approach signal are aligned for a none diverging movement. With these conditions established, the engineer must begin reduction to medium speed (30 MPH) at or before reaching the Approach indication. In addition to reducing their train to medium speed, he must also take action to be prepared to stop at the next signal they encounter.

MEDIUM APPROACH (RULE 286)

The Medium Approach indication is used to indicate to the engineer that the dispatcher has set a power operated turnout for the diverging route, and that the speed on such route can not exceed medium speed (30MPH). It also indicates that only one unoccupied block is ahead. The "diverging route" may be to crossover from one main track to another, enters a siding and/or enters double from single track. In addition to restricting the movement to medium speed they must also take action to be prepared to stop at the next signal he encounters.

In figure 13 the dispatcher has set crossover 9 to the reverse position to cross an eastbound over from track 1 to track 2 at interlocking (B). Signal 12L is displaying the Medium Approach indication. The westbound engineer will proceed through interlocking (B) and over to track 1 not exceeding medium speed. They will then proceed not to exceed medium speed and prepare to stop at the next signal they encounter (i.e. signal 2L).

SLOW CLEAR (RULE 287)

The Slow Clear indication is used to indicate to the engineer that the dispatcher has set a power operated turnout for the diverging route and that the speed on such route can not exceed slow speed (15MPH). It also indicates a minimum of two unoccupied blocks are ahead. The "diverging route" maybe to crossover from one main track to another or enter a siding.

Figure 12 show one common uses for the Slow Clear indication. In this illustration the dispatcher has set crossover 177 to the reverse position to cross an eastbound over from track 1 to track 2. Signal 178R is displaying the Slow Clear indication, and in addition, intermediate signal 1654-1 is displaying the Approach Slow indication to advise the eastbound engineer of the upcoming slow speed restriction. Once the last car of the eastbound train clears the interlocking the engineer may then resume maximum authorized speed.

SLOW APPROACH (RULE 288)

The Slow Approach indication is used to indicate to the engineer that the dispatcher has set a power operated turnout for the diverging route and that the speed on such route can not exceed slow speed (15MPH). It also indicates that only one unoccupied block is ahead. The “diverging route” may be to crossover from one main track to another, or enter a siding. In addition to restricting their movement to slow speed they must also take action to be prepared to stop at the next signal encountered.

In figure 14 the dispatcher set switch 1 to the reverse position for westbound to enter the mainline from the siding at interlocking (A). Signal 2LB is displaying the Slow Approach indication. The westbound engineer will proceed through interlocking (A) and on to the main track not exceeding slow speed. He will then proceed not exceeding medium speed and prepare to stop at the next signal he encounters (i.e. signal 443 @ MP 44.3).

RESTRICTING (RULE 290)

This indication puts much responsibility on the engineer for the safe movement of their train. As stated in most railroad's book of rules, the engineer must " Proceed prepared to stop short of train, obstruction, switch improperly lined, broken rail or anything that may require the speed to be reduced, but not exceeding fifteen (15) MPH." The statement in this rule "but not exceeding fifteen (15) MPH" is only applicable in the best of conditions. If visibility is low or obstructed, the speed must be reduced below the 15 MPH maximum allowable to permit stopping short of an upcoming obstruction. The Restricting indication can be used a number of ways, such as entering a yard, siding or for "back to train" and/or "call on" movements.

STOP AND PROCEED (RULE 291)

The Stop and Proceed indication can only be displayed by an intermediate signal (signals with number plates). Intermediate signals are defined as "A block signal which conveys Stop and Proceed (Rule 291) as its most restrictive indication." This indication (Stop and Proceed Rule 291) informs the engineer that they must stop at the red intermediate signal and then proceed, being prepared to stop short of an obstruction, or anything else that may require them to stop but not exceeding fifteen (15) MPH. This allows for greater flexibility and efficiency in the operation of the signal system. For example, if the dispatcher has two eastbound trains to run, one behind the other (less than one block between each other), these two trains can follow one another safely with the second eastbound train receiving a Stop and Proceed indication (versus a Stop and Stay indication). Otherwise we will have a stop and wait until the train clears the block ahead. By allowing this type of following movement, undue delay to the second train is greatly reduced.

STOP (RULE 292)

Little if any explanation is needed about this indication. One important point that should be explained, in accordance with some railroads (SP, WP, AT&SF, etc.) book of rules. A stop indication may have a plate with the letter "A" attached to the mast or in adjacent proximity to the signal.


Part 5 "The Control Machine"

The control machine provides the important means for the dispatcher to monitor wayside conditions and to initiate the control codes thru push buttons and levers. The control codes are sent to the field equipment via the code line to control the corresponding signals, switches, and electric locks. As with most, if not all aspects of railroading the control machine is as unique as the prototype railroad. Union Switch and Signal Company (US&S) of Swissvale Pennsylvania manufactured hundreds of its type “C” 500 series machines for many North America railroads.

The prototype control machine was constructed of steel. It was approximately 54 inches high and 16 inches deep. At the option of the customer US&S would furnish the machine with or without a desk. These machines were composed of standard sections that were either 30 inch and /or 60 inches long. These sections were constructed so that if the size of the installation requires, a number of these sections could be placed end to end or in a configuration such as rectangular or hexagonal shape, for more ready manipulation. With any of the above arrangements of sections the appearance was that of one integral unit.

On the front of the machine above the desk top are located the control levers, code starting push buttons and indication lamps for both train occupancy, switch, and signal functions. This section of the machine consists of a number of similar rows that are assigned to a specific field location. Each row is uniformly spaced at 2 inch centers throughout the length of the machine. The 30 inch cabinets have 15 rows, and the 60 inch cabinets have 30 rows that start 1 inch in from the cabinet’s edge.

Each row may consists of a two-position switch control lever with two indication lamps directly associated; a three-position signal control lever with three indication lamps directly associated; a spring return type push button or a two-position toggle switch for the control of a “calling on” signal; a spring return type push button for initiating the control code to the field equipment; and a two-position toggle switch for control of the “maintainer call” lamp and/or horn at each field location with one indication lamp associated.

The track occupancy indication lamps (both intermediate and OS track occupancy track lights) are located in the track diagram extending across the control machine above the control levers. A single stroke bell is provided for audibly announcing of train movements at each interlocking and sounding the approach to the CTC territory.

Figure 16, illustrates a standard US&S type CTC machine and the function of each device:



Track Model Board:

The track model board shows a simplified representation of the physical track layout. Solid track indicates that the signal system protects that portion of track. Outlined track indicates non-signal protected track. All controlled signals, switches, intermediate signals, and electric locks are shown on the track model board. Above each interlocking is the timetable station name of that location. Traffic, detector, and intermediate track occupancy lamps are also located on the track model board.

OS Track Light:

The OS or interlocking track light indicates train occupancy with interlocking limits (see part 1 for more information on interlocking limits) i.e., between the interlocking home signals. . A single stroke bell is provided for audibly announcing of train movements through this circuit. When this circuit becomes occupied the train dispatcher will show on his train sheet that this train is “on sheet” (OS) by the location that corresponds with that particular circuit.

A two-position push-pull latching button is provided to cut out this audible indication for the OS track ONLY. This bell also sounds on the “approach” of a train to the CTC territory on each side of the machine. This function can be cutout by uses of a second push-pull latching button or a spring return type push button for “Approach Bell Alarm Cutout”.

Intermediate Track Light:

Intermediate track lights indicate train occupancy between interlockings, on the main and/or side track. They will also illuminate if an automatic electric lock is open and/or a slide detector fence is tripped in the block in question.

Traffic Indication Lights:

When illuminated, these lights indicate the direction of traffic through each block between interlockings. When a Home Signal is cleared into the track section in question the traffic indication light will light and will remain so until the last car clears the OS section of the outbound interlocking. Also if the dispatcher manually codes the signal to stop, the traffic light will remain lit until the expiration of "time", this is part of time locking. Time locking protects against the dispatcher clearing a signal for an approaching train, then putting the signal back to stop and changing the route. When the dispatcher manually puts the signal to stop, the signal system will make him wait a set “time”. While the system is running time the dispatcher cannot change the original route or line any conflicting signals. This is to ensure that the approaching train that may have passed the governing approach signal displaying a Clear (Rule 281) indication has a chance to stop short of the Home Signal that is now displaying Stop (Rule 292). If the approaching train is unable to stop, his route will still be “locked” and free of any conflicting movements avoiding what, without Time Locking may have been a catastrophe (See Part 2 for more information on Time Locking).

Now you may ask why when the signal system is "running time" is the traffic light still lit. Traffic Locking, which is that part of the signal system that ensures that a train does not receive a favorable indication into a block if an opposing route is established, keeps the dispatcher from clearing an opposing signal and prevents him from making a “corn field meet.”

The Association of American Railroads (AAR) guidelines for vital signal circuits for Traffic Locking states “On track signaled for movements in both directions the vital traffic circuits shall assure that the established traffic remains in effect while the track section between interlockings is occupied. Traffic Locking shall also prevent the manipulation of levers or other devices from changing the direction of traffic on a section of track while that section is occupied or while a signal governing movement into that section is running time or displays an aspect for a movement to proceed into that section." In a nut shell, the traffic light remains lit on the dispatcher’s panel because Traffic Locking remains in effect until the expiration of time or until all track sections between interlockings are unoccupied.

Maintainer Call (MC) Toggle Switch and Indication Lamp:

The Maintainer Call (MC) feature was simply a light and/or horn that the dispatcher could activate on the side of a relay house located at each interlocking in question. The dispatcher would flip the maintainer call toggle switch to the “on” position he would then press the code start button to activate this feature. The Maintainer Call light would illuminate on the side of the relay house and if so equipped with a horn it would blow for 8 seconds.

The MC indication lamp below the code start button on the control machine simply repeated the field MC light. After the conversation between the employee and the Train Dispatcher was complete, the dispatcher would manually turn the maintainer call light off. This feature was NOT in any way electrically connected with the phone system on most railroads. One important point though, the term "Maintainer Call" is a bit misleading! Most railroads Book of Rules required ANY employee observing the light lit to immediately communicate with the train dispatcher.

Power Switch or Electric Lock Control Lever and Indication Lamps:

The switch control lever is a two-position 60 degree rotary (left to right) switch that allows the dispatcher to initiate the control code to the field equipment to control the corresponding power or electrically locked switch. The “N” and “R” labels on the control lever plate stands for Normal and Reverse. Normal is the main track route and Reverse is the diverging route, either from one main track to another or from the main track to a siding.

Electrically-locked switch lever control plates are also labeled N and R for normal and reverse and are controlled by the Dispatcher. Normal is lined and locked for the main track and reverse being unlocked and the switch lined by hand for the siding. An electric lock provides a means of locking a manually operated switch with the signal system so that the switch cannot be operated unless traffic conditions permit. In conjunction with the N or R prefix switches and electric locks are labeled with ODD numbers, 1, 3, 5, 7, etc..

When the normal and reverse indication lamps are lit, they indicate that the field equipment has responded to the control code sent by the dispatcher and that the power or electrically lock switch has now assumed the new position so indicated.

Signal Control Lever and Indication Lamps:

The signal control lever is a three-position +/-30 degree rotary (left, center, or right) lever that allows the dispatcher to initiate the control code to the field equipment to control the corresponding signals. The label on the control lever plate “L” and “R” stands for Left (west) and Right (east). In conjunction with the L or R prefix signals are numbered EVENLY, 2, 4, 6, 8, etc. (see figure 17 for the six standard CTC and two standard train order signal control lever plates).

The left and right indication lamps indicate that the wayside signal in the field that corresponds with the lever position selected has responded to the control code sent by the dispatcher and is actually now displaying an aspect greater then stop (Rule292).

The signals normal lamp will only light when all signals controlled by the lever in question are displaying stop (Rule 292) and that they are NOT in the process of “running time”. During the process of running time all three indication lamps associated with the signal control lever will be dark. After the expiration of “time” the signals normal lamp will light.

Call On Button or Toggle Switch:

The Call On button is a push button or two-position toggle switch. This feature is used for “back to train” movements. A good example of a back to train move is, a local freight pulls into a siding at a given station and must do some switching downtown. The conductor cuts the head cars away from his train and proceeds out of the siding and down to the local industries. All good local freight conductors would block their train according to their stops along the line. Once his switching is complete downtown, the light engine must now return to the remainder of his train left in the siding. Basic signal logic tells us that the dispatcher will be unable to clear a signal into the occupied block of the siding. By aligning the switch and signal levers appropriately and pushing and holding the “Call On” button or flipping-up the toggle switch while pushing the code start button, a special control code is sent to the field to override the safety feature in the system of protecting the occupied block in the siding. A restricting (Rule 290) signal aspect is then displayed on the governing wayside signal allowing the light engine to proceed into the siding.

A restricting signal requires that the engineer "Proceed prepared to stop short of train, obstruction, switch improperly lined, broken rail or anything that may require the speed to be reduced, but not exceeding fifteen (15) MPH". In this case, the obstruction is his train which was left in the siding. By requiring the dispatcher to push and hold an additional button or flip an additional toggle switch, there can be no mistake in the dispatchers mind that he is activating this special feature and overriding certain safety features in the system. An important note though for programming logic is that the interlocking limits must be clear and all power switches within the interlocking limits aligned appropriately for the back to train route for this feature to activate.

Code Start Button:

The code start button is a spring return type push button for initiating the control code to the field equipment. The Dispatcher pushes the code start button as the last action needed to initiate this or any code in that column. No command will be sent to the field until this button is pushed. Changing any lever and/or toggle switch position will not cause a change until this action!

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© 2011 - Mike Burgett
Control Train Components - www.ctcparts.com