Rail Signal

Verified Timeline

Lore & Background

Originally, signals displayed simple stop or proceed indications. As traffic density increased, this proved to be too limiting and refinements were added. One such refinement was the addition of distant signals on the approach to stop signals. The distant signal gave the driver warning that they were approaching a signal which might require a stop. This allowed for an overall increase in speed, since train drivers no longer had to drive at a speed within sighting distance of the stop signal. Under timetable and train order operation, the signals did not directly convey orders to the train crew. Instead, they directed the crew to pick up orders, possibly stopping to do so if the order warranted it. The first railway semaphore was erected by Charles Hutton Gregory on the London and Croydon Railway at New Cross Gate, southeast London, in 1841. It was similar in form to the optical telegraphs then being replaced on land by the electric telegraph. Gregory's installation was inspected and approved for the Board of Trade by Major-General Charles Pasley. Pasley had invented a system of optical telegraphy through semaphores in 1822 for the British military, and appears to have suggested to Gregory the application of the semaphore to railway signaling.

In Their Own Story

The introduction of electric light bulbs made it possible to produce colour light signals which were bright enough to be seen during daylight, starting in 1904. The signal head is the portion of a colour light signal which displays the aspects. To display a larger number of indications, a single signal might have multiple signal heads. Some systems used a single head coupled with auxiliary lights to modify the basic aspect. Colour light signals come in two forms. The most prevalent form is the multi-unit type, with separate lights and lenses for each colour, in the manner of a traffic light. Hoods and shields are generally provided to shade the lights from sunlight which could cause false indications. Searchlight signals were the most often used signal type in the U.S. until recently. In these, a single incandescent light bulb is used in each head, and either an A.C. or D.C. relay mechanism is used to move a coloured spectacle (or "roundel") in front of the lamp. In this manner, gravity (fail safe) returns the red roundel into the lamp's optical path. Widely used in the U.S. from World War II onward, searchlight signals have the disadvantage of having moving parts which may be deliberately tampered with. This had led to them becoming less common during the last fifteen to twenty years when vandalism began to render them vulnerable to false indications.

Reader's Guide

Signals have aspects and indications. The aspect is the visual appearance of the signal; the indication is the meaning. In American practice the indications have conventional names, so that for instance "Medium Approach" means "Proceed at not exceeding medium speed; be prepared to stop at next signal". Different railroads historically assigned different meanings to the same aspect, so it is common as a result of mergers to find that different divisions of a modern railroad may have different rules governing the interpretation of signal aspects. For example, stop aspect refers to any signal aspect that does not allow the driver to pass the signal. Signals control motion past the point at which the signal stands and into the next section of track. They may also convey information about the state of the next signal to be encountered. Signals are sometimes said to "protect" the points or switches, section of track, etc. that they are ahead of. The term "ahead of" can be confusing, so official UK practice is to use the terms in rear of and in advance of. When a train is waiting at a signal it is "in rear of" that signal and the danger being protected by the signal is "in advance of" the train and signal. In North American practice, a distinction must be made between absolute signals, which can display a "Stop" (or "Stop and Stay") indication, and permissive signals, which display a "Stop & Proceed" aspect. Furthermore, a permissive signal may be marked as a Grade Signal where a train does not need to physically stop for a "Stop & Proceed" signal, but only decelerate to a speed slow enough to stop short of any obstructions. Interlocking ('controlled') signals are typically absolute, while automatic signals (i.e. those controlled through track occupancy alone, not by a signalman) are usually permissive. Operating rules normally specify that a signal with an abnormality, such as one with an extinguished lamp or an entirely dark signal, must be interpreted as the most restrictive aspect – generally "Stop" or "Stop and Proceed".

Did You Know?

• The first railway semaphore was erected by Charles Hutton Gregory on the London and Croydon Railway at New Cross Gate, southeast London, in 1841.
• Major-General Charles Pasley invented a system of optical telegraphy through semaphores in 1822 for the British military and appears to have suggested to Gregory the application of the semaphore to railway signaling.
• Electric light bulbs made colour light signals bright enough for daylight use starting in 1904.
• Searchlight signals, widely used in the U.S. from World War II onward, have moving parts that may be deliberately tampered with, leading to their decline due to vandalism.
• In North American practice, a permissive signal may be marked as a Grade Signal where a train does not need to physically stop for a 'Stop & Proceed' signal, but only decelerate to a speed slow enough to stop short of any obstructions.