The rail industry generally considers high voltage as above 1kV ac, and at least 1.5kV dc.

Low voltage is considered to be between 50 and 1kV ac and between 120 and 1.5kV dc.

Extra-low voltage is considered less than 50V ac or less than 120V dc.

These are the standards of the International Electrotechnical Commission and its various associated bodies (IEEE, VDE, IET, etc.).

A home signal is an absolute signal that advises a driver if it is safe to enter the station block. The signal may display: 

• two conditions:  danger (red) or clear (green).
• three conditions: danger (red), caution (yellow) or clear (green).

The term home signal applies to more than a station. Home signals protect junctions, points, level crossings, movable bridges, platforms, or block sections.

See also its opposite, the starter signal.

A discipline which studies the effectiveness of communication between humans and machines. 

An acronym for heating, ventilation and air conditioning

Blue writing (sometimes with an underline) in the Web Rail site. You will encounter many of these in our courses and if you click on them they will take you to special places such as documents, web sites or a glossary of terms.

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.