Railway signalling
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Railway signalling is a safety system used on railways to prevent trains from colliding. Trains are uniquely susceptible to collision because, running on fixed rails, they are not capable of avoiding a collision by steering away, as can a road vehicle; furthermore, trains cannot decelerate rapidly, and are frequently operating at speeds where by the time the driver/engineer can see an obstacle, the train cannot stop in time to avoid colliding with it. This necessity was at the base of the establishment of strict guidelines for time keeping and Railway chronometers in 1891 by the general time inspector Webb C. Ball of Cleveland, Ohio, USA.
In Australia, Railway signalling is known as safeworking, perhaps due to the fact that not all safeworking systems involve signals.
Most forms of train control involve messages being passed from those in charge of the rail network or portions of it to the train crew; these are known as 'signals' and from this the topic of train control is known as 'signalling'.
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Timetable operation
The simplest form of operation, in terms of equipment at least, is operation according to a timetable. Everything is laid down in advance and every train crew knows the timetable. Trains can only operate in pre-arranged time periods, during which they have 'possession' of the track and no other train can operate.
When trains are operating in opposing directions on a single-line railroad, meets are scheduled, where each train must wait for the other at a point they can pass. Neither is permitted to move until the other has arrived.
The timetable system has several disadvantages. The first is that there is no positive confirmation that the track ahead is clear; only that it should be clear. This system does not allow for breakdowns and other such problems. The timetable is set up in such a way that there should be sufficient time between trains for the crew of a broken-down or delayed train to walk back up the line far enough to set up warning flags, flares and the explosive devices known as detonators or torpedoes (UK and US practice, respectively) which alert a train crew to a blocked track ahead.
The second problem is the timetable system's inflexibility; trains cannot be added or delayed; trains cannot be rescheduled.
The third is a corollary of the second; the timetable system is inefficient. To give a little flexibility, the timetable must give trains a broad swath of time to allow for some delay. Thus, the line is possessed by the train for much longer than is really necessary.
Nonetheless, this system permits operation on a vast scale, with no requirements for any kind of communication that travels faster than a train. Timetable operation was the normal mode of operation on American railroads in the early days.
In Britain the original form of signalling was performed by policemen with flags. They would allow a train to pass into the next section of line and then wait for a prescribed time, to allow that train to pass through the section, before allowing the following train to pass. This was called time interval working and is still used in some remote parts of the world. The principal drawback to time interval working is that should any mishap befall the first train requiring it to take longer to pass through the section, or even stop dead for some reason in the section, the following driver would have no indication that this section was still occupied. The result was often fatal.
It was for this reason that other methods of signalling were sought. It was the invention of the railway telegraph that really moved safety forward. Now signalmen could let each other know, by bell codes, whether a section of line was clear before allow the next train onto it. It also allowed trains to run at higher speeds as drivers could now have much more confidence that once signalled onto a stretch of line it would be clear.
Timetable and train order
With the advent of the telegraph, a more sophisticated system became possible. The telegraph allows the dissemination of alterations to the timetable, known as train orders. These override the timetable, allowing the cancellation, rescheduling and addition of trains, and most anything else. Sufficient time must be given, however, so that all train crews can receive the changed orders.
Train crews generally receive the orders at the next station at which they stop; or sometimes orders are handed up to a locomotive 'on the run' via a long staff.
Timetable and train order operation was commonly used on American railroads until the 1960s, including some quite large operations such as the Wabash Railroad and the Nickel Plate Road.
Timetable and train order was not used widely outside North America.
Signals
Timetable and train order operation still has some significant flaws, such as an over-reliance on the ability of the crew of a stranded train to let other trains know of the problem, and a general intolerance for human error. When everything goes perfectly it works well, but mistakes are easy and deadly.
Timetable and train order is only suitable for railway lines which carry relatively little traffic, and is unworkable on busy rail lines because it requires great separation between trains. Where this is the case, physical signals need to be used (either mechanical semaphore signals, or - more commonly in the modern era - electric light signals) to show the train crew whether the line ahead is occupied and to ensure that sufficient space is kept between trains to allow them to stop.
Blocks
If two trains cannot be running on the same section of track at the same time, then they cannot collide. This notion forms the basis of most signalling systems.
The rail network is divided into sections, known as blocks. Two trains are not allowed to be in the same block at the same time. A train cannot enter a block until it is permitted, generally by a signal that the block ahead is empty.
On high-speed railways, where trains travel at speeds faster than 200 km/h (125 mph), the trains travel too fast for the driver to see conventional lineside signals, so a system of in-cab signals transmitted by radio is used.
History of block signalling
In the very early days of railways, on double-tracked railway lines, where trains travelled in one direction on the same stretch of track, a means was needed to space out the trains to ensure that they did not collide. In the very early days of railways, men were employed to stand next to the line at certain intervals with a stop watch, these men used hand signals to signal to train drivers that a preceding train had passed more or less than a certain number of minutes ago, this was called "time interval working". If a train had passed the man only a short while ago, the following train was expected to slow down or stop to allow sufficient space to develop between the trains, to prevent a collision.
This system was flawed, however, as the watchman had no way of knowing whether the preceding train had cleared the tracks ahead. And so if the preceding train broke down or stopped for some reason, the following train would have no way of knowing, and collide with it rear-on. Accidents of this type were common in the early days of railways. However, with the invention of the electrical telegraph, it became possible for the station or signal box ahead to send a message (usually a bell ring) back to confirm that a train had passed and that the line ahead was clear; this was called the "block system".
Mechanical semaphore signals replaced hand signals in the early 1840s. When the all-clear message was received, a signalman in a signal box would pull a lever which would move the signal into the all-clear position. This required the placing of signal boxes at regular intervals along the line.
Modern railway signalling
On most modern railways, detection of the train's position on the line and signal changing are done automatically. and coloured-light signals have largely replaced mechanical ones.
The light signals usually mean:
- green: Proceed at line speed.
- yellow: Warning: the next signal is at red.
- red: Stop.
Railway signalling differs from the traffic lights used on roads in that trains cannot stop quickly; for instance, a train travelling at 160 km/h (100 mph) would need several kilometres to stop. Therefore on a railway, trains need to be given advanced warning of a red stop signal so that they can slow down to a speed where they can stop quickly (a yellow signal). Put another way, yellow signals generally do not indicate an impending change to a red signal at the same location. Rather, a yellow signal gives a train advance warning that the following signal will be red.
To achieve this, the railway is divided into 'blocks', where blocks are stretches of track between two signals. Trains are automatically detected when they enter a block, which is done by what is known as a "track circuit".
When calculating the size of the blocks and hence the spacing between the signals, the following has to be taken into account:
- Line speed (the maximum speed the train is allowed to travel)
- Gradient (to compensate for the assistance or otherwise afforded to de acceleration)
- Sighting (the ability of the driver to see the signal)
- Reaction time (of the driver)
The track at either end of the block is electrically insulated, and within the block a small electrical current passes through the track. When a train passes a signal and enters a block, the metal wheels and axle of the train short-circuits the current, which causes a relay associated with the track circuit to itself become de-energized.
When the relay is de-energized, the signal which the train has just passed automatically turns from green to red, the signal behind that one automatically turns yellow, and the signal behind that one turns green.
If any train is following behind, the yellow signal will warn it to slow down in order to stop at the next signal. If, however, the train in front has passed into the next block, the following train will come across another yellow signal. If the train in front is travelling faster than the following train and clears two blocks, the following train will come across a green signal.
The genius of this relay-based track circuit system is its fail-safe mode of operation. If the relay accidentally becomes de-energized, it fails, or the circuit is otherwise disturbed, the track circuit's status and the potential for presence of a train is unknown. In all of these cases, the track circuit causes the associated signal to drop to red.
In the UK a variation of this is used whereby each set of signal lights has four lights in order from top down: yellow, green, yellow, red. The red and green signals are used as described, as is the lower yellow light. The upper yellow light is used to provide a 'double yellow' signal which serves as a warning that the next block is on yellow, thus providing a warning of a red signal a further block in advance.
Double signalling is sometimes used; this is the method used in some areas of New South Wales, Australia. It derives from semaphore signalling. Two sets of lights are displayed, one above the other. The upper light is the condition at the current signal, and the lower is an indication of the aspect being shown the next signal. Some lights have a small lamp at the bottom; this is an indication to continue at low speed.
The double signals indicate:
- green over green: continue
- green over red: caution, next signal at stop
- green over yellow: caution, next signal at green over red
- red over red: stop
- red over red with small lamp lit: low speed, 25 km/h.
The track circuit is also used by signalmen to detect exactly where a train is on a line. In a signal box, a map of the stretch of track the signal box is controlling is usually put up on the wall (called a mimic panel), and when a train enters a particular block, a light representing that block on the map lights up to show the train's location. In the UK, it is usually numbered with the train's headcode so that the signalman can see which train it is. Track circuits are also used to trigger automatic barriers on level crossings (grade crossings in U.S. usage).
Notes on U.S. signalling
U.S. railways used a far greater variety of signalling systems than other countries. There have never been national standards for signal appearance and operation, so each of the hundreds of rail lines developed its own signalling techniques. Further, there is a good deal of bidirectional track in the U.S., and also a fair amount of "dark track" with no signalling equipment installed.
Notes on UK signal colour order
The green above and red below colour order of British railway signals (as opposed to the red above and green below of traffic lights on roads) derives from the oil-lit colour lenses of the old semaphore signals. The design of semaphore signal ultimately settled on by most of the British railway network had a raised arm for 'go' and a lowered arm for 'stop', thus ensuring a fail safe situation that if the arm failed, gravity would set it to stop. Since the lights for night use were at the post end of the semaphore arm, this meant that in order for the correct colour to show, it was necessary to place red at the bottom and green at top.
Another reason is snow. Each signal lamp has a hood to shade it against glare from the sun. If snow builds up on top, the lamp above can be obscured. Putting the most important lamp, the red, at the bottom means that there is nothing below it for snow to build up on. It will always be visible even if the others are obscured.
The requirement of road signals that a stop light should be visible over a queue of traffic and as far back as possible (thus demanding an order with red above) does not really apply to railways with a block signalling system as there should be no traffic queuing within a block, there being only one train in a block at a time. Also, red need not be at the top for maximum visibility over distance as the driver of a train will already be expecting a red signal, having passed a yellow one in the previous block. This level of advance warning is altogether missing with road signals.
The red light of a multi-lamp signalling system is also positioned such that the red signal - the most important is nearest to the line of sight of the driver. Be it noted that ground signals are in reverse with the red signal at the top (again nearest to the drivers line of sight).
For higher speed lines, 4-aspect signalling is used, where a second yellow lamp is used above the green lamp. The sequence of signals is this instance is: Red - Stop, Single Yellow - next block is occupied and next signal is at Red, Double Yellow - next signal is at Single Yellow, Green - next signal is at Double Yellow or Green. There are many variations on this basic theme depending on the track layout (whether there are junctions, cross-overs, stations, bay platforms etc. and at interfaces with areas with 3-aspect signalling or 2-aspect signalling.
See also
External links
- Railroad Signalling: North American Signaling and Safety (http://www.lundsten.dk/us_signaling/index.html)