Signalling Systems

The entire signalling system can be classified into two main categories.

(a) Mechanical signalling system

(b) Electrical signalling system

In addition to these two main categories of signalling systems, a solid-state signalling system is also in use. Each system of signalling comprises four main components.

(a) Operated units such as signals and points

(b) A transmission system such as single- or double-wire transmission or electrical transmission

(c) Operating units such as levers and press buttons

(d) Monitoring units such as detectors, treadle bars, and track circuiting

The comparison between mechanical and electrical signalling based on these four broad components is given in Table 31.8.

Table 31.8 Comparison of signalling systems




Operated units Signals

Mechanically operated signals

Coloured light signals with two-

as per lower quadrant or upper

aspect, three-aspect or four-aspect

quadrant signalling



Mechanically operated points;

Electrically operated points (by

locking with the help of point

converting the rotary movement of

locks, stretcher bars, and

electric motors into linear push or


pull); locking with the help of slides and solid rods.

Level crossing

Manually operated swing leaf

Electrically operated lifting


gate or mechanically operated lifting barriers



Single- or double-wire

Electrical transmission through


transmission to the requisite

overhead wires or underground

points by means of rods or double wires


Operating units

Hand levers with a range of

Push buttons, rotary switches, or

500 to 2000 m used in collaboration with single-wire or double-wire lever frames

electrical signalling equipment

Mechanical interlocking with

Interlocking through

tappets, etc.

electromagnetic switches known as relays or solid-state switching devices


Monitoring of points with the

Monitoring with the help of direct


help of detectors; monitoring

current track circuits, alternating

of the passage of trains using a

current track circuits, electronic

treadle, which is an electromechanical device

track circuits, axle counters, etc.

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