Railway signalling

This susceptibility is exacerbated by the enormous weight and inertia of a train, which makes it difficult to quickly stop when encountering an obstacle.

In the UK, the Regulation of Railways Act 1889 introduced a series of requirements on matters such as the implementation of interlocked block signalling and other safety measures as a direct result of the Armagh rail disaster in that year.

Nonetheless, this system permits operation on a vast scale, with no requirements for any kind of communication that travels faster than a train.

North American practice meant that train crews generally received their orders at the next station at which they stopped, or were sometimes handed up to a locomotive 'on the run' via a long staff.

In the very early days of railways, men (originally called 'policemen', which is the origin of UK signalmen being referred to as "bob", "bobby" or "officer", when train-crew are speaking to them via a signal telephone) were employed to stand at intervals ("blocks") along the line with a stopwatch and use hand signals to inform train drivers that a train had passed more or less than a certain number of minutes previously.

The absolute block system came into use gradually during the 1850s and 1860s and became mandatory in the United Kingdom after Parliament passed legislation in 1889 following a number of accidents, most notably the Armagh rail disaster.

Under a permissive block system, trains are permitted to pass signals indicating the line ahead is occupied, but only at such a speed that they can stop safely should an obstacle come into view.

In these cases, trains must proceed at very low speed (typically 32 km/h (20 mph) or less) so that they are able to stop short of any obstruction.

On timetable, train order, and token-based systems, blocks usually start and end at selected stations.

The system depends on knowledge of the precise location and speed and direction of each train, which is determined by a combination of several sensors such as radio frequency identification along the track, ultra-wideband, radar, inertial measurement units, accelerometers and trainborne speedometers (GNSS systems cannot be relied upon because they do not work in tunnels).

Moving block setups require instructions to be directly passed to the train instead of using lineside signals.

This has the advantage of increasing track capacity by allowing trains to run closer together while maintaining the required safety margins.

Centralized traffic control (CTC) is a form of railway signalling that originated in North America.

Electrical circuits also prove that points are locked in the appropriate position before the signal protecting that route can be cleared.

On most railways, physical signals are erected at the lineside to indicate to drivers whether the line ahead is occupied and to ensure that sufficient space exists between trains to allow them to stop.

The earliest types comprised a board that was either turned face-on and fully visible to the driver, or rotated so as to be practically invisible.

Whilst it is normal to associate the presentation of a green light with a safe condition, this was not historically the case.

A green light subsequently replaced white for 'clear', to address concerns that a broken red lens could be taken by a driver as a false 'clear' indication.

This method has the disadvantage that the driver may be unfamiliar with the required speed over a junction onto which they have been diverted due to some emergency condition.

Under route signalling, the aspects necessary to control speed do not exist, so a system known as approach release is often employed.

The driver brakes in accordance with the caution aspect, without necessarily being aware that the diverging route has in fact been set.

There is also a system of flashing yellows used in the UK that allows trains to approach a diverging route at higher speed.

The driver's route knowledge tells them permissible speed across the diverging junction, and they will begin to slow the train upon seeing the two flashing yellows.

Others include audible and/or visual indications inside the driver's cab to supplement the line side signals.

In-cab safety systems are of great benefit during fog, when poor visibility would otherwise require that restrictive measures be put in place.

Safety systems are also important in urban rail where it is impossible to see around corners in subway and metro tunnels.

On-board and wayside computers can track trains around tight corners at higher speeds ensuring safety.

In the early days of the railways, signalmen were responsible for ensuring any points (US: switches) were set correctly before allowing a train to proceed.

The concept of the mechanical interlocking of point switches, signals and other appliances was introduced to improve safety.

Modern interlocking systems and subsystems allow and prohibit certain point switch positions to enhance train safety.