Transit Technology: Fundamentals and Subsequent Development

This is a series of articles by Sky Train Corporation, discussing the foundation and development of some familiar details of transit technology. The text reflects to some extent the separate functions that the different components can serve, a way  of  slotting the ideas into compartments. For example, the size and design of vehicle bodies depends upon the load-carrying capacity of the trucks and track, the size of body that the clearances of the right of way can accept, but also on the nature of the traffic to be carried, such as short haul commuter traffic, long haul passengers, airport feeders, and the various natures of freight traffic. Given these constraints, body design follows basic principles.

Signals, the why and the wherefore

1)     What is “Safe Speed?”

Excessive speed is dangerous! Everybody knows how easy it is to lose control of a vehicle, even on a straight stretch of road, when the zigzags just build up, if speed is beyond the limit of safety. There are other ways this danger shows up. Going round a curve too fast can lead to skidding sideways, perhaps into another vehicle. Or the vehicle may roll over, off the road and into the ditch. For steel wheel vehicles on a railroad track, zigzags and skidding sideways cannot happen, but the sideways force might force the outer wheel to climb the rail, so that would be a derailment. Roll can build up until the train rolls off the track. Excessive speed on the curve might allow the vehicle to roll over outwards and create a pile-up.

It is possible to use railroad signals to control the speeds of trains, to keep them below the speed limits, but that is not where they came from. Railroad signals came from the fact that the stopping distance for a train at speed is longer than the distance that the engineman can see ahead. Safe speed is to be able to stop a train before it might hit any obstruction, that might be another train on the track ahead, that the engineman has not even seen yet!

The need for signals developed when railroad speeds were wound up until the stopping distances became longer and longer. Whether on a highway or on a railroad, safety is being able to stop short of any obstruction ahead. In the streets, it is left to the driver of an automobile, a bus, or a streetcar, to judge what is the safe speed to follow the vehicle ahead, or approaching a curve. This is known as "Driving on sight". When trains were driven this way, speeds had to be limited, because the distance where an obstruction could be seen and recognized might be less than half a mile, or round a curve. A safe speed that would allow a train to stop in this distance would be much slower than the capabilities of a modern railroad.

2) Faster trains:

So for faster trains, an engineman needs some kind of signal, that will tell whether the track is clear for the train to proceed at full speed. This is to afford rear-end protection for  the train ahead, so that the following train will not run into it.

When the track is clear, for the train to continue at speed, all the signals show the aspect "Clear", for proceed. Protection for a train is provided by changing a signal aspect behind it to show "Stop", as soon as the train has passed it. At first, this was done manually by a flagman on the ground or by a towerman, who had wires controlling the signals, without having to be out beside the track. As soon as a train passed the signal, the towerman would put it into the stop position, and hold it there until that train had moved into the protection of the next signal beyond that one, then it could be cleared to let a following train move by. This is known as "Block signaling".

When track circuits were introduced, it became automatic for a train to set its own protective signals.

Track circuits are based on the two rails in a block section being insulated electrically from one another. A small voltage is connected between the rails, at the leaving end. If there is no train on that section of track, the voltage passes down the whole length of the rails, and activates a relay at the entry end. The fact that the signal reached the entry end proves that there is no train in the section. The arriving voltage energizes the relay and allows the signal there to show clear, so that an approaching train is cleared to enter that track section. This serves a side benefit, in proving that the rails are electrically continuous, so there are no broken rails in the block. As soon as the train enters on to the section, the steel wheels of the train create a short circuit between the rails, so that the voltage no longer reaches the relay at the entry end. The relay drops out, the signal turns to red, and a following train knows that it should not enter the section that that signal is protecting. In effect, the trains report their presence in the signal sections, by shorting out the rails.

Unfortunately, it is not good enough, just to show a red stop signal to an approaching train. By the time an engineman can see that it is showing stop, it is far too late to stop the train before passing it. There has to be a different signal far enough back, giving warning that the next signal is at stop. The signal immediately behind the train must show stop, but the same information has to be sent on to the next signal behind it, to show yellow, for "Caution". That is a warning that the approaching train should slow down, ready to stop at the next signal, because it is red. The distance between the yellow and the red signals must be long enough for the train to be able to stop in time from the permitted speed.. That has to be the distance required to stop the train within the yellow section. The closer together the signals are, the slower would be the permitted speed. This is the most simplistic arrangement of automatic block signaling.

Braking distances vary from light-rail trains, that can stop in a few hundred metres, up to heavy freight trains, that can require 3km or more.

3) Mixed traffic

It is easy to design a signal arrangement, for a train service, where all the trains are made up of the same type of rolling stock, and all have the same braking performance. There would be a clear relationship between the lengths of the signal blocks, and the speeds the trains can be permitted to operate at.

It is not quite so simple, when different trains have different braking performance. Effectively, the distances available for braking to stop at a signal are fixed, once the signals are installed at track side. So the speeds the various trains may be permitted to operate must be different, always to ensure that they can brake to stop within the block length. The operators of each train must know what are the speeds they can operate at, and never exceed them, for safety. Still, all signals show clear, if there is no need to prepare to stop.

If the block lengths have been designed for heavy trains, with limited braking performance, then faster trains with better brakes would be penalized. More trains could be run closer together, if the blocks had been shorter. This becomes important, only if traffic levels demand trains closer together, than the simple blocks can accommodate. In that event, the arrangement of signals can be elaborated, by putting an additional signal within the length of the braking section. The outer signal becomes a double yellow, and the new signal a single yellow. Then the heavy train must react to the double yellow, to have the full block length to stop in. The lighter train can ignore the double yellow, and continue to roll at full speed until the single yellow tells whether it must prepare to stop within the now-shorter block length.

Signal aspects can instruct the train what is maximum speed for operation in the next section. This may be for traversing crossovers, or running into a section where the available braking distance is less than adequate. This is known as "Speed signaling".

A more specific elaboration comes in, when transit trains should be closed up, when one of them is standing at a station platform. The time a train stands at a station is called the "Dwell" time. In transit service, the objective is to operate the trains at the closest practicable headway. A second train should enter the platform, as soon as the first train has moved on. This can be achieved, by providing several signals at the approach to the station, each signal imposing a slower speed on the train. Then the second train can be almost touching the train ahead, when it stops.

The importance then is to allow the second train to run into the station as closely as possible behind the train that is leaving. For this, some subway lines have another signal about the middle of the platform. This signal shows red, until the departing train clears the next signal at the leaving end of the platform. As soon as the rear of the leaving train clears this signals, the signal at the entry to the platform can show "Restricted speed", so the second train can pass it slowly. In short time, the leaving train gains enough speed, to allow the signal at the middle to clear, and the second train reaches the full length of the platform

4) Automatic operation.

The first voltages used in the track circuits were direct current, from lead/acid batteries. Later, the circuits were powered from the main electrical supply, and were alternating current. Then an antenna suspended above the rail in front of the train can detect  the presence of currents in the track. In the absence of any current in the rails, it is reasonable to assume that there is already a train in the block, so this train should make an emergency stop.

The next step was to put different codes on the circuit currents, intended to command different signal aspects at the entry end of the block. At that stage, the different aspects were to tell the train the speed at which it should move as it entered the block. When it became clear that a train could read the track codes, it was logical to have a new signal in the cab with the driver. Naturally, this is called "Cab signaling".

Finally, if the speed commands could be detected in the cab, then these codes could control directly the speeds of the train movements, without any involvement of the driver. As a train moves along the track, each block gives a specific code, that tells the train to adjust to a new speed, and if necessary, to stop in the block. The total absence of code must be interpreted as "Stop in emergency". An emergency stop is too sharp for normal service operations. To avoid rough stops like this for an ordinary service stop, usually, there is a continuing code that says "Stop", In the absence of a train, this code reaches the end of the block, and proves the continuity of the rails.

An arrangement like this allows total automation, so there is no need for a driver to run the train. There are some subway and light rail systems operating in public service, where there are no crew members on the trains. Often there are representatives of the operating company circulating on trains and in stations, to provide a human presence, to supervise all aspects of the service, to answer questions from the passengers, and to fulfill a policing function.

The special value of total automation is that it allows operation of frequent short trains, since crew costs do not relate to the number of trains on line.

5) Centralized traffic control

With track relays being controlled by the presence or absence of trains in the sections, it was a reasonable next step to transmit this information back to a central control room. A controller could have a lighted diagram in front of him, showing the status of all the blocks on his territory, which blocks were occupied by the trains, and to observe their progress as they moved across the territory. Commands could be sent out to the signals and switches, to set the routes and control the speeds of the trains. All commands are electronically linked, to ensure no conflicting commands can go out to the field, that might create an unsafe condition.

This led to a valuable increase in carrying capacity of the rail lines. Trains could be routed into sidings to allow other trains to move safely, and trains in different directions could be switched from one track to another, according to the way the traffic levels demanded.

This is the origin of centralized traffic control, or "CTC", as it is known.

6) Moving block.

Recognizing that the safe condition requires that all trains must have a full braking distance ahead of them, to be able to stop before the next obstruction, some progress has been made to constantly report exactly what is the distance to the next stopping point ahead. The speed of the train is controlled to keep just outside this limit. Then if the command comes to stop, the distance is adequate for safe stopping.

In effect, the train always carries a clear space ahead of it, sufficient to stop in time. This is exactly the same as an automobile driver, keeping the closest safe distance behind the next vehicle ahead. This would not be a fixed block, such as is created by having signals mounted at fixed points along the track. This is the principle of "Moving block". This is complicated and expensive to achieve on a rail line, and has not found wide applications. There have been some experiments using satellites for radio positioning.