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

(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

Component	Mechanical	Electrical
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	signalling
Points	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
	detectors	pull); locking with the help of slides and solid rods.
Level crossing	Manually operated swing leaf	Electrically operated lifting
gates	gate or mechanically operated lifting barriers	barriers
Transmission	Single- or double-wire	Electrical transmission through
systems	transmission to the requisite	overhead wires or underground
	points by means of rods or double wires	cables
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	Monitoring of points with the	Monitoring with the help of direct
units	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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