One item in railroading that is not often in the magazines is a discussion of signals and how they work on the prototype. What is often said by the modeler is “I operate in the dark,” or with CTC, or ABS, without much discussion of what these are and what they do. Perhaps it is best to make the analogy that these are the traffic signals that railroads use. However, it is not a perfect analogy.
Signals are the tools used by dispatchers to manage the flow of traffic over a rail line. Dispatchers can dispatch by train orders or with centralized traffic control (CTC). Train orders are used in dark territories, or rail lines without any signals. Trains travel from one station or passing siding to the next based on orders given by the dispatcher through station agents or others. With signals the trains do not need to use train orders to travel from one place to the other, because the signal will tell the engineer what he needs to do. Centralized traffic control (CTC) allows the dispatcher to let the engineer run the train without stopping for orders. It allows for quicker and more efficient running of trains by controlling interlockings. Automatic block signaling (ABS) does the same thing for the traffic flow but with block signals.
There are two types of signals: block and interlocking. A block signal is a signal which controls a block of territory on a rail line. The signal will have only one head or light on it. Its job is to control the distance between points. Therefore, the color of the signal governs the speed of the train. Green will allow the train to travel to either its maximum speed or the maximum speed of the line, whichever is less. It also indicates that the next block is clear. Yellow is a restrictive indication which means that the train must travel at a medium speed and that the next block is occupied. Therefore, the train must be prepared to stop at the next signal. Remember that blocks are as long as the operations department thinks it will take a train to stop within it.
Red means that the train must stop-sometimes. This is because there are two types of block signals: absolute and permissive. An absolute signal means that the train must stop and not proceed past the signal (absent special limited circumstances) until the aspect changes. A permissive signal means that the train must make a stop but can then proceed into the next block at a restricted speed, always prepared to stop within half the engineer’s sight distance. One special type of permissive signal is a signal which is on a grade. This permissive signal allows a heavy, slow train to proceed past the signal without having to stop and restart the train. A permissive block signal will have a number plate or a plate with the letter “P” or “G” (for grade) on it.
Automatic block signals are block signals which are joined by electrical circuits to provide a signal indication. A block signal at A will display a red, yellow, or green signal depending on whether the next block signal at B is showing red, yellow or green. If a train is between signal A and signal B, then the signal at A will show red. If a train is between signal B and signal C, then B will indicate red and A will indicate yellow. If a train is between signal C and signal D, then C will show red, B will show yellow and C will indicate green. This insures that the trains are appropriately spaced for safety.
An interlocking signal is a signal which governs an interlocking plant. An interlocking plant is the joining of two or more tracks. Signals exist at these points to control movements through these plants and to provide for safety for the trains and their crews. These are the signals that a dispatcher controls with CTC. They are marked by having multiple heads or signal lights. A red signal at an interlocking plant is always considered to be absolute. A train cannot move through an interlocking signal which is red. The top signal head indicates whether a train will be allowed to proceed through the interlocking on the main and, if so, at what speed. The second signal indicates whether the train will be allowed to move through the interlocking on the diverging route and, if so, at what speed. The bottom head on a triple headed signal protects movements though inferior diverging routes.
The top signal means the same as a block signal. Red means stop. Yellow means proceed at medium speed prepared to stop at the next signal. Green means go. If the top signal is red, the maximum speed for moving through a diverging route is medium. If the second head is green, this is a medium clear and the train can move through the interlocking at medium speed and the next signal is either clear (green) or approach (yellow). If it is yellow, (medium approach) the train can move through the diverging route at a medium speed but must be prepared to stop at the next signal, and if it is red, the train must stop. If the top and second heads are red and the third head is green, this is a slow clear, which means that the train can proceed through the interlocking at a slow speed on an inferior diverging route. If the signal is yellow, a restricting signal or a call on indication, the train can proceed on the inferior diverging route at a restricted speed prepared to stop. Red again means the train must stop.
A call on indication allows a train to enter an occupied plant to make switching moves. It is also used to move trains through an interlocking when the mechanisms governing the interlocking are broken, to allow trains into unsignaled track, to allow trains to move from unsignaled track onto the interlocking plant and to allow a train to enter one-way track against the flow of traffic.
The prototype will also use dummy heads on its signals to indicate that the signal is an interlocking signal. The dummy head is always red. The dummy head is the signal head for the track that it is physically impossible for a train to take. For instance, when two tracks meet, one a main line, the other a secondary main, trains approaching from two of the three directions cannot physically take another path. A train approaching the switch can continue on the main or diverge to the secondary. This will need to be a two headed signal with all signals working.
The train on the main approaching from the other direction cannot take the diverging route. Therefore, the bottom signal is unnecessary and a dummy head can be used to note that it is an interlocking signal. If the train is coming from the secondary line to the main, it can only go in one direction; therefore, the top head is a dummy.
The dispatcher controls the interlocking plants and the ABS functions in between these two areas. The dispatcher then has a control panel with the interlockings marked on it, but not the block signals because he does not control these. In addition, the distances between the interlockings are not noted by similar distances on the control panel. The distances between the two areas are more or less uniform. Therefore the dispatcher must be aware of the distances between the interlockings.
This is the way that the prototype signals its lines. It can be modeled on our own pikes and incorporated in the way we run operations. Unfortunately, I cannot provide the information about building the circuits because I do not understand electronics. I need someone to explain electronic things to me–real slowly. But please look at Richard Schumacher’s articles to get started.
Until the next time, I hope all the signals you see are green over red.