What kinds of signals (semaphores, lamps, etc.) does IR use?
IR uses several kinds of signals. Semaphore signals have generally given way to colour-light signals.
Semaphore signals are the older style signals 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...
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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.
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 introduced in 1928 but were slow to take off. In recent years many older semaphore signals have been replaced by colour-light signals.
Position-lightsignals 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.
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.