Stray signals, frequencies, currents or voltages (from the railway and from local industries), may adversely affect signalling equipment. Engineers employ a number of strategies to ensure the equipment is immune to (not adversely affected by) these unwanted triggers.

An impedance bond is a track side mounted device that is connected across the running rails. The bond allows continuity of the traction return current and an earthing connection for the wayside structures (such as gantries) without introducing any imbalance in the track circuits.

It is possible for equipment to be in one position or state, but indicating that it is in another. For example, a set of points might indicate they are set in the normal position but are in fact in the reverse position.

A system is said to be in correspondence when the position of the equipment is the same as that indicated.

When a system's position or state is not the same as indicated, it is said to be out of correspondence.

In-cab signalling uses visual and sometimes auditory devices inside the driver's cab to advise the driver of the condition of the track ahead. 

In-cab signalling is a modern replacement for trackside signalling (colour light or semaphores).

In-cab signalling is a component of ERTMS.

Electrical track circuits are isolated and defined from each other by the use of insulated rail joints (IRJs). The insulated joints can be of a mechanical or glued variety.

A mechanical IRJ is assembled on-site with a number of metal and insulating components.

A glued IRJ is where two short lengths of rail are joined in a factory by superior bonding techniques. The glued  joint, about 1-2 metres in length, is carried to the site and welded into place.
 

Primarily, interlockings prevent the setup of conflicting routes at places such as junctions or crossings. An interlocking is designed so that it is impossible to clear a signal for a train to progress unless the route ahead is proved to be safe.

An interlocking is a safety-critical signalling device. Modern interlockings have many additional functions incorporated such as Automatic Route Setting and Automated Signalling which depend on the safety-critical data contained in interlockings. 

An interlocking can be of the following types:

• mechanical, 
• electro-mechanical (relay) or
• electronic (computer based interlocking or CBI)

The Institution of Railway Signal Engineers, known more usually as the IRSE, is an international organisation, active throughout the world. It is the professional institution for all those engaged or interested in railway signalling and telecommunications and allied disciplines. The IRSE website can be found here.

The IRSE conducts a licensing scheme, enabling signal engineers to register their skills and develop their careers. A broad description on the IRSE Licensing Scheme can be found here.