Railway Mechanical signals
The oldest forms of signal displayed their different indications by a part of the signal being physically moved. The earliest types comprised a board that was either turned face-on and fully visible to the driver, or rotated away so as to be practically invisible. These signals had two or at most three positions.
Semaphore signals were patented in the early 1840s by Joseph James Stevens, and soon became the most widely-used form of mechanical signal, although they are now decreasing in number. The semaphore arm consists of two parts: An arm or blade which pivots at different angles, and a spectacle holding colored lenses which move in front of a lamp in order to provide indications at night. Usually these were combined into a single frame, though in some types (e.g. “somersault” signals in which the arm pivoted in the center), the arm was separate from the spectacle. The arm projects horizontally in its most restrictive aspect; other angles indicate less restrictive aspects.
Semaphores come in “lower quadrant” and “upper quadrant” forms. In lower quadrant signals, the arm pivots down for less restrictive aspects. Upper quadrant signals, as the name implies, pivot the arm upward. Either type may be capable of showing two or three indications depending on the application. For example, it was common in the United States for train order signals to point the arm straight down to indicate “Proceed”.
The color and shape of the arm is commonly varied to show the type of signal and therefore type of indication displayed. A common pattern was to use red, square-ended arms for “stop” signals and yellow “fishtail” arms for “distant” signals. A third type with a pointed end extending outward (in the opposite direction from the fishtail shape) may indicate “proceed at restricted speed after stopping” (and indeed, stopping itself is often waived for heavy freight (“tonnage”) trains already moving at slow speed).
The first railway semaphore was erected by Charles Hutton Gregory on the London and Croydon Railway (later the Brighton) at New Cross, southeast London, in the winter of 1842-1843 on the newly enlarged layout also accommodating the South Eastern Railway. The semaphore was afterwards rapidly adopted as a fixed signal throughout Britain, superseding all other types in most uses by 1870. Such signals were widely adopted in the U.S. after 1908.
Initially, railway semaphores were mounted on the roof of the controlling signal box, but gradually a system of wires and pulleys controlled through mechanical linkages was developed to control the signals at a distance. Signal boxes became controllers of interlockings, and came to be known as interlocking towers or simply signal towers in the United States, while retaining the name “signal box” in the United Kingdom. The signals protecting the station itself came to be called home signals, while signals some distance away giving advance warning came to be called distant signals.
Mechanical signals may be operated by electric motors or hydraulically. The signals are designed to be fail-safe so that if power is lost or a linkage is broken, the arm will move by gravity into the horizontal position. For lower quadrant semaphores this requires special counterweights to cause the arm to rise rather than fall; this is one of the reasons for the widespread switch to upper quadrant signals.
In the U.S., semaphores were employed as train order signals, with the purpose of indicating to engineers whether they should stop to receive a telegraphed order, and also as simply one form of block signalling. Mechanical signals worldwide are being phased out in favour of colour-light signals or, in some cases, signalling systems that do not require lineside signals (e.g. RETB).