Railroad signaling operates in the space of the contradictory requirements for a railroad: The desire to be able to move trains as fast and as often as possible and the desire for train movements to be safe. If trains stand still, safety is at maximum, but the railroad is useless. On the other hand, if trains move freely as the trainmen desire, trains will derail and collide every now and then. So, for our railroad to be safe and useful at the same time, a compromise between these two extremes must be made.
The main safety hazards for the train to derail or for trains to collide. Derailment can happen due to
Collisions can happen as
On all secondary trackage (yards, spurs etc.) all train movements are conducted at Restricted Speed, meaning that trains shall move slow enough as to be able to stop within half the line of vision, keep a lookout for other trains and for switches not properly aligned. This is considered to be safety enough, as the low speed will minimize consequences of any mishaps.
On Main Tracks (tracks so designated in the timetable), it is usually desirable that trains can move faster, and for that other measures have to be taken. These measures involve a number of strict operational rules, and in many cases the addition of a signal system.
Of the derailment hazards, faulty track or train is mainly handled through inspections of equipment and is only peripherally a subject for the signaling and operations system.
Switches that are misaligned for a train movement may cause the train to traverse the switch at a too high speed (see below) and to head onto another track with risk of collision with other trains. Therefore, all main track switches shall be locked in their normal position, unless they are in use. Locking in unsignaled territory is usually a padlock, combined with rules instructing personnel to lock switches. Switch indicator lamps or switch stands are usually provided to enable trains to read switch positions at speed. On signaled tracks, the signaling system will supervise the switches and display a signal aspect reflecting the state of the switch. Switches may also be open, i.e. that the blades are not in either end position. This involves a high risk of derailment. The above measures also apply for open switch detection/prevention.
Overspeed is sought prevented through the timetable listing the permissible track speed anywhere on main tracks. But switches present a special problem, since speed limits are usually different between the two branches. On non-signaled tracks, switches can in general only be traversed through their normal position at speed. Signals, on the other hand, can indicate permitted speed on the tracks ahead, alternatively the route that the train is to take.
Risks of head-on and rear-end collisions are handled much the same way, though with a tendency to focus on head-on collisions due to their more serious consequences. The basic idea is to separate trains in space, i.e. that only one train may be on a given section of track at any one time. In practical terms this rule is softened so that trains may share tracks if they move at restricted speed.
Train separation can be implemented in a number of ways. In unsignaled territory trains must be authorized by manual ways (radio permits, timetable and train orders or one-train-per-track), all relatively slow procedures with a high safety but a low traffic throughput. In signaled territory, signals take over parts of, or the whole, train separation task. Being automatic, and with the signal indications being a fast means of communicating to the trains, a signal system can considerably boost traffic capacity.
The last source of collisions, misaligned switches, is another aspect of the derailment prevention in switches and with the same precautions.
As described above, a signal system can handle many safety aspects as well as speed up train operations. Being a purely technical system, it can also be made highly reliable, and thus much safer than its human counterpart.
In the USA, "signaled territory" means tracks equipped with an Automatic Block Signaling system (ABS). The ABS ensures basic train safety by
The ABS does not in its basic form include any means for controlling train movements, this still has to be handled by similar means as in unsignaled territory, though some precautions can be relaxed in ABS territory.
The basic concept of ABS is that the track is divided into Blocks; sections that are protected by Block Signals. A block signal is a signal that is so controlled by the ABS that the following (somewhat simplified) basic requirements are met:
1st and 2nd requirements address static conditions in the block - the block signal is simply prevented from displaying "Approach" (or better) if a switch is not positively detected to be properly aligned, or if a track is detected to be occupied by a train. 3rd requirement addresses dynamic conditions, that even if 1st and 2nd requirements are met, it must be ensured that trains are slowed down so they can obey the restrictive block signal. Please note that a block signal does not have to display "Stop" if its block is occupied or a switch is not aligned - "Restricting" requires trains to keep a lookout for these conditions.
Trains take a long distance to stop - a train going full speed takes over a mile to do a controlled stop. Since signals are rarely visible that far away, it is necessary to warn trains in advance of signals requiring a slow-down or a stop. The common way to do this is to let a signal not only indicate the condition of the first block, but also include information of the following block (and sometimes a third block, depending on speed and local conditions):
Apart from providing a warning distance of at least a braking distance, the ABS must also cope with dynamic issues. It two trains run against each other on the same track, they must be warned at latest when they are the aggregate of their stopping distances apart:
The safety zones ahead of the train can be implemented in a number of ways, which are outside the scope of this document.
On lines with 2 tracks, each track can be assigned a fixed direction of travel, the so called Current of Traffic. Each track will only be signaled for trains moving With the Current of Traffic, while a (rare) movement Against the Current of Traffic. Lines so equipped are denoted Double Track lines. Double track ABS does not provide safety zones against opposing trains.
An Interlocking is an area with controlled signals and controlled switches, in which signals and switches are interlocked in such a way that
signals cannot display a permissive aspect unless switches are aligned properly for the train move
signals cannot simultaneously display permissive aspects for conflicting train moves
An interlocking typically controls a number of power controlled switches, and allows trains to move from one main track to another, enter or exit sidings, yards etc.
In practical terms, the main functions of an interlocking are:
|
|
|
|
|
|
|
|
|
|
Interlockings can be stand-alone "islands" in otherwise unsignaled tracks, but usually interlockings are in ABS territory. Recalling the basic ABS requirements, the interlocking functions outlines above fulfill these within the interlocking, but dependencies to the ABS outside interlockings also need to be established.
Where the surrounding lines are double track ABS, signaling through the interlocking is reflects the current of traffic. Since moves against the current of traffic are rare, signals governing these moves into the interlocking are usually dwarf signals. Dwarf signals are less expensive than "high" signals but at the same time they can only display Slow speed aspects, limiting train speeds through the interlocking to Slow speed.
The interlocking will usually be designed to provide the following possible signal aspects:
|
|
|
|
Where full flexibility of the trackage is required, main tracks are signaled for both directions. These installations are done according to Traffic Control System (TCS) rules; rules that allow trains to be dispatched and run by signal indication alone. TCS rules are often referred to as Centralized Traffic Control (CTC) rules, but the correct term is TCS *)
TCS describes the rules and requirements for running trains on signal indication alone. CTC is a signaling system where interlockings are controlled from a central office, instead of from local interlocking towers. TCS is a requirement for CTC, but TCS can also be implemented with locally controlled towers or a mixture between CTC and local towers. Interlockings in CTC territory are often referred to as Controlled Points (CP).
In TCS territory, the tracks between interlockings are equipped with Traffic Locking. Traffic locking locks the direction of travel when an interlocking clears a signal to that track. The traffic locking stays in place as long as the signal is cleared or there's a train occupying the traffic locked track.
Traffic locking locks all opposing signals to their most restrictive aspect, and blocks adjacent interlockings from clearing a signal towards that track.
Note that while TCS tracks are usually equally equipped with signals for both directions, this is not a requirement. In multiple track territory, the individual tracks are usually predominantly used in one direction of travel, making it feasible to reduce the number of signals for the unusual direction of travel.
The following is an example of the signaling on single track TCS. The example starts with Train 2 waiting for a meet at Left interlocking. The meeting Train 1 has just arrived but the dispatcher has not yet cleared the signal for Train 2. The track towards Right interlocking is still set for direction right-to-left after Train 1, but since the track is unoccupied and no signal is cleared from Right interlocking, traffic locking is not active:
The dispatcher requests the signal cleared for Train 2. Left interlocking requests the line to Right interlocking set for that direction of travel, and once the direction of travel flips left to right, the interlocking locks its route, again making the ABS activate its traffic locking. The signal now clears for Train 2 - "Medium clear" as the switch only permits Medium speed and the next blocks are free:
As soon as the front of Train 2 passes the signal, the signal changes back to "Stop". The switch is still locked as long as the train occupies the interlocking, and traffic locking on the line is also active:
When Train 2 clears the interlocking, the route unlocks and the switch is released. Traffic locking towards right interlocking is maintained by the presence of the train:
As the train continues, the automatic block signals drop to "Stop and proceed", then clear again. All opposing signals display the "Stop" aspect. The dispatcher clears the signal through right interlocking:
Train 2 continues, and the dispatcher clears a signal at Left interlocking for a train to follow Train 2:
Train 2 clears Right interlocking, releasing its route. Traffic locking between Left and Right interlockings is still active due, to the route cleared from Left interlocking:
The dispatcher Fleets the signal at Left interlocking. Fleeting a signal disables its route release function, automatically clearing the signal for a following train. In essence, a fleeted signal behaves like an automatic block signal. After Train 3 clears Left interlocking, the signal changes to "Stop and proceed":
As the train moves along, the dispatcher clears signals for it at Right interlocking:
In the preceding text, only the standard use of signal aspects has been explained. It has been assumed that signals were placed braking distance apart, and that speed restrictions were derived only from switches. This is, however, not always the case. A number of factors can lead to other configurations of the signal aspect sequences that a train will meet. In many cases a problem has multiple solutions, and the chosen solution can be a result of railroad policy the decision of the person in charge or determined by standard solutions from the signal equipment vendor. None of these decision factors are constant, and on most railroads a wide variety of solutions to the same problems can be found. In the following text some of these alternatives are outlined.
On higher speed mainlines the increasing length and weight of trains over time has increased the braking distance. Since resignaling a line is an expensive task, the cheaper solution of increasing the advance signaling distance has instead been preferred. Some railroads have added another signal aspect named "Advance approach", meaning that the train must prepare to stop at the second signal ahead:
Note: PRR did not use the indicated "Advance approach" aspect, see below
Since the number of possible signal aspects is somewhat limited, other railroads instead chose to use the "Approach medium" aspect:
When departing a track only permitting Medium speed, the normal "Medium clear" aspect can be used. If a train has to maintain Medium speed until clear of Left interlocking, it cannot reach a much higher speed before reaching the next signal at "Approach". Short trains may be able to reach a higher speed, but their braking distance is shorter, leaving this arrangement safe.
When approaching a Slow speed signal, the train will see "Approach medium", followed by "Approach slow", since the braking distance to Slow speed is almost the same as to a full stop:
In some situations signals need to be located at less that braking distance. This is particularly often the case around yards and passenger terminals, but also where multiple junctions are close. The solution is also here to provide the advance signaling at least a braking distance away. In the example below, a railroad crossing and a junction are located near each other, with separate signals. The block between signals 26 and 12R is regular length, as is the block following signal 16R. while the block from 12R to 16R is short. The block after 16R is assumed to be at least regular length.
The block between 12R and 16R, however, is so short that is only provides a safe braking distance if the train is going at Slow speed when entering the block. The following signal aspect sequence is displayed as signals clear through the interlockings:
Note: An intuitive alternative solution would be for signal 12R to display the "Slow approach" aspect, but is a train accepts the "Slow approach" aspect at 12R, and later 16R changes to "Approach" or "Clear", the train would have to proceed at no more than Slow speed until the whole train has cleared the interlocking following 12R.
Most signal rules are incomplete with respect to providing a signal aspect for all thinkable situations. Referring to the PRR rules, there is no "Slow clear" that can be displayed from high signals, and the rules lack aspects like "Advance approach", "Medium approach Medium" etc. Many of these aspects would only be necessary in very few, and since the number of possible combinations of signal lamps is limited, the railroads instead live with a less than optimum signaling.
In the example below, a train is to cross over to the neighboring main track at no more than Medium speed, then at next signal divert at Slow speed. The correct signal aspect from signal 12R would have been "Medium approach Slow", but this aspect does not exist in the rulebook. Alternatively 12R could display "Medium approach" which with good visibility of signal 14R would be almost as good, but this aspect does not exist either (it has later been added to the rule book). Next alternative is "Slow clear", but again this does not exist from high signals (and a dwarf signal would permit only Slow speed on all routes, unacceptable for the straight routes). So stepping down one more step, signal 12R will have to display "Slow approach". This means that the train will have to go at Slow speed all the way from 12R and be prepared to stop at 14R,. If visibility of 14R is poor, the train will come almost to a standstill:
If smooth operations require proper advance signaling of 14R, an extra signal (18R) can be added after the Medium speed crossovers, allowing 12R to display the "Medium clear" aspect. This is, however, only possible if the distance between 18R and 14R provides safe braking distance from Medium Speed to Slow speed:
Text, Images, HTML: Carsten S. Lundsten.
