Signalling systems

The Indian Railways makes use of colour signal lights, but in some remote areas of operation, the older semaphores and discs-based signalling (depending on the position or colour) are still in use. Except for some high-traffic sections around large cities and junctions, the network does not use automatic block systems. However, the signals at stations are almost invariably interlocked with the setting of points (routes) and so safety does not depend on the skill of the station masters. With the planned introduction of Cab signalling/Anti collision devices the element of risk on account of drivers overshooting signals will also be eliminated.

Coloured signalling makes use of multi-coloured lighting and in many places is automatically controlled. There are three modes:

• Two aspect signalling, which uses a red (bottom) and green (top) lamp
• Three aspect signalling, which uses an additional amber lamp in the centre
• Four (multiple) aspect signalling makes use of four lamps, the fourth is amber and is placed above the other three.

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 utilisation. At the same time, slower trains can also be run closer together on track with multiple aspect signals.
Semaphores make use of a mechanical arm to indicate the line condition. Several subtypes are used:

• Two aspect lower quadrant
• Three aspect modified lower quadrant
• Multiple aspect upper quadrant
• Disc-based: These signals are located close to levers used to operate points. They are all two-aspect signals.