Classification of Signals

Railway signals can be classified based on different characteristics as presented in Table 31.1.

Table 31.1 Classification of signals


Basis of classification



Communication of message in audible or visual form

Audible: Detonators Visual: Hand signals, fixed signals, etc.


Signalling the driver to stop, move cautiously, proceed, or carry out shunting operations

Stop signals, shunt signals, speed indicators


Reception or departure signals

Outer, home, starter, and advanced starter signals



Meant for special purposes

Calling-on signals, repeater signals, speed indicators, etc.

Figure 31.1 shows the further classification of audible and visible signals. Table 31.2 lists the minimum signal requirements of various classes of stations.

Classification of signals
Fig. 31.1 Classification of signals

Table 31.2 Signals required at stations

Classification of station

Minimum requirement of signals


A class

Warner, home, and

An outer signal can be provided


after obtaining special permission

B class

Outer and home

In multiple-aspect upper quadrant (MAUQ) areas, distant home and outer signals are provided

C class

Warner and home

In MAUQ areas, the warner signal is replaced by a distant signal

Further details regarding the different types of stations are furnished in Chapter 26.

Audible Signals

Audible signals such as detonators and fog signals are used in cloudy and foggy weather when hand or fixed signals are not visible. Their sound can immediately attract the attention of drivers. Detonators contain explosive material and are fixed to the rail by means of clips. In thick foggy weather, detonators are kept about 90 m ahead of a signal to indicate the presence of the signal to the drivers. Once the train passes over the detonators thereby causing them to explode, the driver becomes alert and keeps a lookout for the signal so that he/she can take the requisite action.

Visible Signals

These signals are visible and draw the attention of the drivers because of their strategic positions.

Hand signals These signals are in the form of flags (red or green) fixed to wooden handles that are held by railway personnel assigned this particular duty. If the flags are not available, signalling may be done using bare arms during the day. In the night, hand lamps with movable green and red slides are used for signalling purposes.

Fixed signal These are firmly fixed on the ground by the side of the track and can be further subdivided into caution indicators and stop signals.

Caution indicators These are fixed signals provided for communicating to the driver that the track ahead is not fit for the running the train at normal speed. These signals are used when engineering works are underway and are shifted from one place to another depending upon requirement.

Stop signals These are fixed signals that normally do not change their position. They inform the drivers about the condition of the railway line lying ahead.

The stop signals normally used on railways are semaphore signals, coloured light signals, and other such signals as explained in subsequent sections.

Objectives of Signalling | RAILWAY ENGINEERING - Contents | Fixed Signals