The problem: trains cannot swerve
A train cannot steer around an obstacle, and its steel wheels on steel rails need a long distance to stop. A driver who sees another train ahead often cannot halt in time. Early railways therefore needed methods to ensure that two trains were never in the same stretch of track at the same time, and that drivers would know what lay ahead. Signals are essentially a way of passing that information.
Timetables and the time-interval method
The earliest railways relied on timetables and on lookouts. Staff, sometimes called railway policemen in Britain, stood along the line and gave hand signals with flags by day and lamps by night. A common system used the time interval: a train could leave a station only after a set number of minutes had passed since the previous one. This worked while traffic was light and trains ran at similar speeds. It failed when a train broke down or stopped, because the following driver had no way of knowing and the signalman had no way of warning him.
Fixed signals began to replace hand signals in the 1830s. Early examples included balls or discs raised on poles, and lamps with colored glass. Which railway introduced which device first is not always certain, and different lines used different conventions, which created confusion for crews who ran across more than one railway.
Semaphores
The semaphore signal, with a movable arm on a post, became the classic fixed signal. An arm held horizontal meant stop, and an arm lowered or raised at an angle meant proceed. The design is often credited to Charles Hutton Gregory in the early 1840s, though many railways modified and promoted their own versions. The semaphore had a practical advantage: if a cable failed, a weighted arm could fall back to the stop position. Lamps with colored lenses were fitted so the arm could also be read at night.
Colors for lamps varied before they were standardized: red generally meant stop, green caution or clear in different systems, and white clear in early British practice. Over time, red for stop and green for clear became common across many countries, partly because white lights could be mistaken for stray lamps.
The block system
The most important change was the block system. The line is divided into sections called blocks, and only one train is allowed in a block at once. A signalman at the start of each block will not let another train enter until he has been told the previous train has left. This replaced “time apart” with “space apart”.
The electric telegraph made this possible. Telegraph lines ran along railways from the 1840s, and signalmen could send bell codes and messages to the next box ahead. The system with the strictest rules is usually called absolute block working. In Britain, W. R. Sykes and others developed devices that made it harder to give a clear signal by mistake, and Edward Tyer designed telegraph instruments and tablets used on single-track lines.
On single lines, where trains run in both directions, an extra safeguard was needed. Tokens or staffs, physical objects handed to the driver, provided authority: only the train holding the unique token could occupy the section.
Interlocking
Signals alone are not enough if a signalman can set points to one route and a signal to another. Interlocking connects the levers so that conflicting signals and points cannot be set together. The system developed in Britain in the 1850s, with John Saxby among those who patented early frames. Mechanical locking inside a signal box made it physically impossible to pull a lever for a clear signal if the points were set against the route.
Accidents and regulation
Many safety changes followed accidents. In Britain, a collision in the Clayton Tunnel in 1861 involved a mix-up between signaling messages and helped push railways toward more reliable block working. Another, at Armagh in Ireland in 1889, killed many people, among them children on an excursion, after a train stalled on a gradient and a following section of the train rolled back. It contributed to the Regulation of Railways Act of 1889, which required block working on passenger lines, interlocking of points and signals, and continuous automatic brakes on passenger trains. Automatic air brakes, based on designs by George Westinghouse in the United States, worked in coordination with signaling because a brake that applies itself if the train separates is a second layer of protection.
Track circuits and electric signals
The track circuit, developed in the United States in the 1870s, used the rails themselves as part of an electric circuit. A train’s axles short out the current and tell the signal system that a block is occupied, without relying on a person to say so. Colored electric lights, which were also a model for road traffic signals, gradually replaced arms in many places during the twentieth century, and automatic block signaling allowed signals to change by themselves as trains moved. Later developments included cab signaling and automatic train protection, which display information to the driver or apply the brakes if a signal is passed. Today many networks use computers and radio-based train control, but they rest on the same basic idea: separate trains by space, and make it hard to make a dangerous mistake.