Signalling Systems

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Signalling Systems

Q. Why are multiple aspect signalling systems used? What was wrong with the older systems which had two aspects?

Multiple aspect signals, by providing several intermediate speed stages between ‘clear’ and ‘on’, allow high-speed trains sufficient time to brake safely if required. This becomes very important as train speeds rise. Without multiple-aspect signals, the stop signals have to be placed very far apart to allow sufficient braking distance, and this reduces track utilization. At the same time, slower trains can also be run closer together on track with multiple aspect signals.

Q. What kinds of signals (semaphores, lamps, etc.) does IR use?

IR uses several kinds of signals. Colour-light signals are the standard. In the past, semaphore signalling was in use almost everywhere, but these have been steadily replaced over the last couple of decades.

Colour-light signals are assemblies of lamps that indicate different aspects by means of different colours of lamps that are lit. Colour-light signals were first introduced in 1928 but were slow to take off.

Semaphore signals are the older style signals that were seen widely throughout the country, where each signal has an assembly with an arm mounted on a mast, where the arm can move through two or three different positions at different angles, each position providing a distinct signalling aspect. Very early in India's railway history, two-position lower-quadrant semaphore signals were the most prevalent. Around the 1930s, however, the introduction of American style power signalling equipment in some areas resulted in three-position upper-quadrant signalling being introduced as well, although both systems continued in use for many decades afterwards. It is not clear when distant signals were introduced.

Position-light signals are assemblies of lamps where the signal aspect is indicated not by colour but rather by the combination of the lamps that are lit.

Disc signals are in the form of a vertical disc with a pattern such as a bar painted on it, which rotates about its centre to different positions to indicate different signal aspects. These are usually mounted on poles but may be close to ground level.

Target signals have a vertical disc (or two parallel vertical discs) which can rotate about a vertical axis so as to present the disc either face-on or edge-on to an observer along the track. Usually a lamp is provided behind the disc (or between the parallel discs) which is visible only when the discs are oriented edge-on. The centres of the discs usually also have lamps. The two aspects of this type of signal are indicated by the two orientations of the discs. This type of signal is almost always at ground level.

Note: In the following, ‘on’ refers to that position of a signal which shows its most restrictive indication (in accordance with IR's terminology). However, we use ‘clear’ for the position that shows the least restrictive indication instead of the word 'off' because the latter is used by IR to refer to any signal position other than the on position.

Q. What types of signalling systems are used on IR?

IR uses several forms of signalling. In IR manuals, reference is made usually only to four types of systems: Multiple Aspect Colour-Light, Lower Quadrant semaphore, Modified Lower Quadrant semaphore, and Multiple Aspect Upper Quadrant semaphore. But in practice there are some variations in the kinds of colour-light signalling seen, so for ease of analysis, the following classification is used here. (Abbreviations in parentheses given for ease of reference in the text that follows.)

  • Two-aspect Lower Quadrant semaphore signalling (2LQ)
  • Modified Lower Quadrant semaphore signalling (MLQ)
  • Multiple Aspect Upper Quadrant semaphore signalling (MAUQ)
  • Two-aspect Colour-Light signalling (2CL)
  • Three-aspect Colour-Light signalling (3CL)
  • Four-aspect Colour-Light signalling, normally known just as Multiple Aspect Colour-Light signalling (MACL)

These are explained in detail in the section on aspects and indications of signals.

In addition to these, there are some in-cab warning systems (AWS), and of course flag/lamp/hand signals for emergency use.

Colour-light Signals

Q. What are the systems of colour-light signalling used by IR?

There are three systems of colour-light signalling in use. (In IR terminology, the term Multiple-Aspect Colour-Light signalling includes both 3- and 4-aspect signalling, and 2-aspect signalling is usually treated as a variant of 2-aspect semaphore signalling. Hence the classification below is not the same as IR's.)

  • Two-aspect colour-light signalling: In this, each signal has two lamps (one above the other). The higher of the two is a green lamp, and the lower one is a red lamp. The green lamp when lit indicates clear (the proceed indication), and when the red lamp is lit, the signal is said to be in the on position, displaying its most restrictive indication.
  • Three-aspect colour-light signalling: In this, each signal has three lamps arranged vertically. The top one is green, the middle one yellow, and the bottom one is red. The red and green lamps indicate indications as in the 2-aspect system, and the yellow lamp shows the caution indication.
  • Four-aspect colour-light signalling: This is also known just as multiple-aspect colour-light signalling (MACL or MACLS) and adds another yellow lamp to the 3-aspect system. The additional yellow lamp can be placed above the green lamp in a 4-lamp signal. In this case, the lower yellow lamp alone is lit to show the caution indication, and both yellow lamps are lit to show the attention indication. Alternatively, a different kind of 3-lamp signal may be used (e.g., for distant signals), where the top and bottom lamps are yellow and the middle one is green. Again, both yellow lamps light up to indicate the attention indication.

Special signals such as repeaters may have other combinations, e.g., two lamps, green above yellow.

The obvious advantage of colour-light signalling over semaphore signalling is the higher reliability of electrical control over the signals compared to the mechanical means for operating semaphore signals. Colour-light signals do not suffer from distance limitations as semaphore signalling does (exception: powered semaphore signalling), allowing signal controls to be placed conveniently together even if the signals themselves are far away. In addition, the electrical circuitry naturally allows for monitoring, interlocking, and detection of failure conditions, all of which are achievable but far less reliably with mechanical means in semaphore signalling.

Semaphore Signals

Note: Other than one or two isolated sections (these too are slated to be converted to colour-light soon), semaphore signalling is not in use on IR anymore. Much of this information is historical.

Q. What are the systems of semaphore signalling used by IR?

Lower Quadrant

In IR's lower quadrant system (Two-aspect Lower Quadrant) the semaphore arm can only be in two positions. The horizontal on position shows the most restrictive indication (requiring the train to stop or slow down or proceed with caution depending on the kind of signal), and a lowered position where the semaphore arm is at about 60 degrees or more from the horizontal shows the clear or proceed indication allowing a train to go past the signal.

The 2-aspect Lower Quadrant system suffers from a couple of disadvantages. The principal disadvantage is that the driver of a train must be prepared to bring the train to a full stop when the warner is at caution and the home signal is at danger. To address this, often warner signals are moved further back to provide sufficient distance from the home signal for braking the train to a full stop. The second disadvantage with the 2LQ system is that the indication of the warner signal is not explicit. When the warner is at caution, it may indicate that the home signal is at danger, or that the train will be received on a loop line, or that there is a speed restriction of some sort ahead.

These disadvantages are addressed with the Modified Lower Quadrant system. In this, warners and distant signals (as in MAUQ, see below) are both used. The distant signals have only two aspects, Proceed and Caution. The distant signal is provided at an adequate distance to the rear of the Home signal, and a combination Home and Warner signal is provided 180m from where the block section ends. There is no difference in the placement or nature of the last stop signal. MLQ was found in the Kharagpur - Vishakhapatnam and a few other sections. It was not widely adopted as it is complex in working and provides no advantages over the competing multiple-aspect upper quadrant signalling system (see below) which also came into use and became far more commonly used on all important sections of IR.

Early versions of semaphores used in the lower quadrant system suffered from a potentially dangerous flaw, which is that in case of a mechanical failure, the semaphore arm was likely to drop by gravity into the clear position. This was guarded against in later versions by having the spectacle end of the semaphore be considerably heavier to provide a counterweight to the arm. Generally speaking, fail-safe operation to ensure the signal shows its most restrictive aspect when the signal wire is broken is ensured by arranging counter-weights or adjusting the balance of weights between the semaphore arm and the spectacle appropriately, in both lower-quadrant and upper-quadrant signalling.

Upper Quadrant

Properly, Multiple Aspect Upper Quadrant. In this system there are three signal positions. The 12 o'clock position is clear or proceed, which gives a train permission to go past the signal without stopping. An intermediate position (at 45 degrees to the vertical) is the attention or caution indication; the meaning depends on the kind of signal. The horizontal position, where the semaphore arm is horizontal, the on position, is the most restrictive indication of the signal; it may require the train to stop, or to proceed with caution, etc., depending on the kind of signal.

More Notes

In all semaphore systems, as the semaphore arm moves from one aspect to another, the end that is close to the signal mast and which has coloured glass disks (‘spectacles’) fixed to it moves in front of a lamp, changing the colour of the lamp seen at night. Most of these lamps are electric lamps, but oil lamps were common earlier.

Semaphore signals are set up so that when viewed from the part of the track for which the signal is intended, the semaphore arm extends to the left of the mast on which it mounted. This, in addition to the colours of the semaphore arm (which are different on the front and back), provides a visual cue to distinguish between the signals meant for different directions of the track.

Assemblies of 2 or 3 or more semaphore signals on the same mast structure occur to indicate divergent routes. Usually, one of the signals is placed higher than the others, to indicate the ‘main’ line; the signals to its left or right are somewhat lower, and apply to signals to branches diverging to the left and right. Signals may be at the same height if the divergent routes are all of the same importance. Such multiple signal assemblies are seen for stop signals (home, starter, etc.) and also for distant signals (pre-warners).

Q. What are ‘single-wire’ and ‘double-wire’ signalling?

‘Single-wire’ apparatus, as the name implies, utilizes a single wire or cable connecting the signal lever at the cabin or elsewhere where the signal frame is located, to the actual semaphore mechanism on the signal post. Operating the signal lever to take the signal off causes the transmission wire to be pulled, moving the semaphore arm to the required aspect. To reverse this and change the signal aspect to a more restrictive one the signal lever is moved back, and the semaphore arm moves back because of gravity acting on the semaphore mechanism (in some cases there may be appropriate counter-weights for this). In single-wire transmission, a signal can be pulled for up to 900m. A gain stroke wheel may be inserted at the foot of the signal lever to increase the lever stroke, or a so-called ‘facile stroke lever’ may be provided. In these cases the distance over which the signal can be pulled may be as high as 1080m.

In ‘double-wire’ transmission, the wire that operates the semaphore loops around a drum or pulley at either end. Therefore, when the signal lever is moved in either direction, it exerts a positive pulling force to move the semaphore arm. Counter-weights are not necessary in this case. Signals can be pulled over a distance of 1600m in this case.

Q. What are drooping signals?

In single-wire transmission, heat causes the transmission wire to stretch or shrink, and this can result in an incorrect indication of the signal aspect. For instance, the most restrictive aspect may end up being below the horizontal in upper-quadrant signalling on a hot day — this is termed a drooping signal when the angle is more than 5 degrees. Wire adjusters are provided to compensate for temperature variations. The problem is minimized in double-wire transmission as there is positive movement of the wire in each direction and the wire remains in tension at all times.

Signal Indications

Q. What indications do signals show and what do they mean?

The most common indications shown by various signals are the following:

  • Stop: This requires a train to stop dead and not pass the signal except under special instructions or emergency procedures. (Stop signals may be passed after halting and waiting in automatic block territory – usually 1 min. during the day & 2 min. during the night.) This indication is also known as Danger.
  • Caution: This allows a train to proceed past the signal with caution (at reduced speed), being prepared to stop at the next signal. It can mean that the next signal is at Danger, or that the track ahead has speed restrictions.
  • Attention: This allows a train to proceed past the signal, being prepared to slow down to an appropriate speed for the next signal. It means that the next signal may be at Caution, or may guard a divergence which requires reduced speed (in which case a stop signal at the divergence will indicate the route for which points are set).
  • Proceed: This allows the train to proceed past the signal without slowing down or stopping.
  • Proceed Slow: This indication, shown only by calling-on signals, allows a train to pass the signal at slow speed after stopping, being prepared to stop short of another train or an obstruction on the same track.
  • Proceed Slow for Shunting: This indication, shown by shunting signals, allows movement past past the signal with caution for the purposes of shunting. This is the most common indication used when a shunt signal is pulled off, and in fact most shunt signals can only show this indication (other than Stop).
  • Proceed for Shunting: This indication, shown by shunting signals, allows movement past the signal for the purposes of shunting, at speeds higher than allowed with the indication Proceed Slow for Shunting. This indication is not widely used, and appears in 3-aspect position light shunt signals.


Q. What are running signals and subsidiary signals?

Running signals are the normal signals that control the movement of regular trains. Subsidiary signals are those that control other movements such as shunting, or which provide additional information (repeater signals, points indicators, etc.).

Q. How do signals refer to specific lines in the case of diverging and converging routes?

Colour Light Signals

For colour-light signals, a junction route indicator or directional type route indicator is commonly used to indicate diverging routes. This consists of an additional set of 5 lunar white lamps in a row at an angle, attached to the main signal. The angle of the junction route indicator corresponds in a rough manner to the angle made by the diverging route. When these additional lamps are lit, they indicate that the signal applies to a diverging route. Otherwise, the signal is taken to apply to the main route.

More than one junction route indicator may be attached to a signal, in the case of facing points where more than two routes diverge, although it is rare to see more than 3 or 4 such indicators (6 is the maximum). The junction route indicator corresponds to a ‘feather’ in UK railway terminology (some IR officials and railfans call these ‘mickeys’ (because the resemble the face of Mickey Mouse)). Junction route indicators are used where the number of diverging routes is smaller and where high visibility is a requirement.

In some cases, especially for home signals at stations that have many platforms, or at routing signals guarding approach to a lot of diverging routes, a theatre route indicator may be provided. This usually indicates the route (or road as it is sometimes termed) with a numeric display. The numerals may be formed using a 7x5 dot-matrix lamp assembly (the multi-lamp route indicator, MLRI), or with lamps lit behind stencils indicating route numbers (the stencil type route indicator, STRI).

There are also projector type route indicators which project the numeral on to an illuminated screen or plate. For all of these, a route indication is always provided, even for the main line, in contrast to the directional route indicators which remain unlit for the main line.

For a signal guarding departure from a station, a theatre route indicator may rarely have ‘M’ or ‘ML’ to indicate ‘main line’, and ‘B’ or ‘BL’ to indicate a ‘branch line’; similarly ‘L’ or ‘LL’ for ‘loop line’. The visibility of these is not as good as that of junction route indicators, hence they are used mainly near or within station limits where speeds are not high, but where the number of diverging routes may be large.

Normally, signals for multiple converging routes are placed on separate posts, and in some cases on a bracket post or signal gantry or bridge. In rare cases more than one signal may be placed on the same post, in which case the topmost refers to the leftmost route, and successive signals below it refer to successive routes to the right.

Semaphore Signals

Multiple signals may be mounted on a signal assembly (bracket post, signal gantry, etc.) to provide signal indications for diverging routes. The signals from left to right correspond to the diverging routes from left to right. If one of the routes is the main line, the signal for it is usually placed higher than the others (the maximum permissible speed applies for running through on it; speeds must be lowered for the divergences).

For instance, a very common combination is for three stop signals to be mounted together, with the middle one being placed higher and providing the indication for the main route, whereas the signals on the left and right of it provide indications for the branches on either side. If all routes are of equal importance, all signals are at the same height. (‘Equal importance’ in practice means all the routes allow the maximum permissible speed for the section.)

In rare circumstances, one can find multiple signals placed on the same mast one above the other; in such a case, the convention is that the highest one refers to the leftmost divergence, and successive signals below it refer to successive routes to the right.

The same convention applies for converging routes (top-to-bottom is left-to-right). Although diverging routes can share a single signal (with a route indicator in colour-light signalling), converging routes never share signals; a separate signal is provided for each line.

Q. In semaphore systems, what did rings, bars, etc. found on some signals mean?

Stop signals controlling the approach to goods yards or goods-only lines had a black ring fixed to the end of the semaphore arm. No corresponding indication is provided in colour-light territory. Similarly, semaphore signals controlling lines for dock platforms had a black semicircle (in the shape of a ‘D’) fixed to the end of the semaphore arm. Again, no corresponding indication is provided for a colour-light signal.

The ‘X’ fixed to the end of the semaphore arm meant the signal was decommissioned and need not be adhered to. In colour-light signalling, a white ‘X’ over the entire bracket is provided to indicated a decommissioned signal.

Q. What does it mean when a colour-light signal does not face along the tracks but points away?

Colour-light signals that are not in use (just set up but not yet commissioned, or in the process of being decommissioned) are often turned to point away from the tracks, so that it is clear to all locomotive drivers that the signal is not in service. Otherwise, it would be treated as an active signal that is malfunctioning (lamps burnt out), which would require trains to follow special procedures for passing malfunctioning signals.

Q. What is the purpose of the white lamps fitted to the rear of signals?

Signals that face away from the signal cabin are provided with back lights to enable the signal operator to see the aspect of the signal. Normally a single white lamp is lit when the signal is on, and no lamp is lit otherwise. For stop signals that can show the Attention indication, two white lamps are visible in the on aspect and no lamps otherwise (distant signals that can show Attention have only a single back light).

Q. Sometimes a signal pole is observed to carry one signal at normal height and another much higher up; what are those? Or, what are Co-acting Signals?

A co-acting signal is a duplicate signal provided on the same mast as a stop signal, which always shows the same indication as that stop signal. The purpose of such a co-acting signal is to allow a continuous unobstructed view of the signal indication from all positions where a driver might need to observe it, in cases where an over-bridge or other obstruction might block the view of a signal from some locations if there were only one instance of the signal provided on the mast.

Typically, one of the signals is fixed very high up on a mast and the other one much lower down, so that one or the other is always in view from all positions along the tracks as it is approached. Although theoretically more than two such co-acting signals could be provided on a single pole, this is never seen in practice.

Q. What does ‘ahead’ or ‘behind’, ‘advanced’ or ‘retarded’, or ‘front’ or ‘rear’ mean when referring to a track or signals?

All orientation terms used when talking about track, points, signals, stations, etc. are given from the point of view of the driver of a train looking in the direction that the train is moving. Thus, a signal may be ahead of him or behind him. A signal or station that he is approaching is referred to as being in front, and one that he has passed is said to be in the rear. An ‘advanced’ starter signal is one that is further ahead than the starter signal, and so on.

Q. What is a ‘fixed signal’?

A fixed signal is any signal that is permanently erected at a location. The term is used to distinguish normal signals and indicators from hand or lamp and flag signals, detonators, flares, bells, and other special-purpose methods of signalling.

Q. How is failure of signals guarded against?

Signal installations are designed as far as possible for fail-safe operation, which means that any failure should leave the system in a state where dangerous train movements are not allowed. For instance, in case of a failure detected at a panel interlocking installation, all signals controlled by it are designed to revert to On. Similarly, a failure detected in the block control circuit at the Starter signal causes all signals to the rear guarding the approach to the block section switch to On, and notification is sent automatically to the control centre or signal cabin.

LED panels are used extensively for the greater safety they afford (since several LEDs on a panel can fail without compromising the safety of the signal).

In the past, signals themselves had two-filament bulbs, or two-bulb assemblies for each lamp, to provide redundancy in case of a filament burning out. Where incandescent bulbs were used, the filaments were kept warm even when the lamp was off, through the passage of a small current which prevented thermal shocks on switching on the lamp and thereby reduced the chances of failure. A current relay also detected the current flowing in the signal lamp in its different states, and this allowed detection of a failed lamp. (Even in the days of kerosene lamps for signals, a bimetallic thermal contact strip was used to detect the heat of the lamp and notify the signalman if the lamp was extinguished.)

The signals are also frequently examined and individual LEDs (or panels, bulbs in the past) replaced in a pre-emptive manner.

Back lights for electric signals today (and small slits in the rear of kerosene-lamp signals in days gone by) allow the signalman or stationmaster to see the states of the signals at a station. Where visibility limits the use of back lights, the signal aspect is repeated in the signal cabin or (at small stations) in the station master's office.

In the latest instances of signalling control by means of interlinked stations failure-detection circuits are provided for each track circuit and signal circuit with notification to the signal control centres in case of problems.

Signal installations are usually powered by independent power supplies (DC) that are driven by battery installations that are charged from the regional grid (state electricity board's supply). All the failsafe equipment and the signals themselves also have emergency fail-over to backup battery sets that keep the signals and points working in case of power failure. Most stations also have diesel generator sets to continue charging the batteries in case of power failure.

For details of the meanings of various signals, continue to section on signal aspects and indications. Or continue to the section on train working, block system, etc.