A signal box is a structure which houses train signaling and switching devices. Historically, such mechanical devices were often located on exposed areas of track, forcing the operator to move around the tracks to set switches and signals. Eventually, railways recognized that housing these devices in a structure would be safer and more pleasant, and the signal box or signal cabin was developed. Today, train signaling and switching are highly centralized, making signal boxes much less common.
Signal boxes usually house an interlocking, which is actually a very complex mechanical or electrical system which is used to control tracks and trains. The idea behind an interlocking is that it will not allow conflicting settings to be entered, thereby greatly reducing the risk of an accident. For example, if tracks have just been switched to allow a train onto track A, the interlocking will not allow the operator to set a signal indicating that it is safe for a train approaching on track A to proceed.
Interlocking can include just the signals, ensuring that an operator does not inadvertently set conflicting signals, or it may include the track switches as well, depending on the design of the system. In the early days of railroading, the interlocking in the signal box was entirely mechanical, with a set of levers used for switching and signaling. Today, the system is often controlled electronically, which also allows for remote control, because an operator no longer needs to be close to the tracks to set signals and switches.
Signal boxes also house communications, which would have been telegraphs in the early days of rail, but are more likely to include telephones, radios, and computer systems today. The communications system allows signal boxes to convey information to each other, which can be important for safety. Train delays, alterations in schedule, news about accidents, and other information can be sent along the communications system.
A railway may opt to maintain a signal box as a historical curiosity. Railroad buffs often enjoy visiting signal boxes and seeing the equipment. A signal box is usually named for the area it is located in, which may be a station, a crossroads, or a remote section of track. As with other relics from the heyday of railways, signal boxes are interesting historical sites to visit, and some people feel that they are worth preserving, even if railroads are no longer using them or if the tracks they once controlled have been removed to make way for development.