In the early days of railroading, trains were run by "line of sight." Speeds were so low that drivers could stop before hitting an obstacle. Relatively effective, trains began to go faster, operate at night, and through single-tracked segments, and more robust form of train control was soon needed.

The Liverpool and Manchester was likely the first railroad to adopt a form of time-based authority [1]. The railroad was divided into segments (what we would today call blocks), and each segment was patrolled by a railroad policeman. When a train would enter a block, the policeman would set a timer to the scheduled amount of time for the train to traverse the block and prevent another train from passing until the timer had expired.

This worked well enough in favorable conditions, but increasing capacity was impractical as shortening blocks would require more railway policemen, and the system relied on trains never taking long than their scheduled time to go between policemen. Breakdowns or slow-downs would create an unsafe situation: the timer having expired, the policeman would let a train into the occupied segment, risking a rear-end crash. As railroads looked to shorten the "intervals"—effectively reducing padding—crashes increased. Modifications were made, but real advances in safety did not come until station to station communication via the telegraph was introduced [1].

By 1839, the telegraph signalling had been implemented on the Great Western Railway, also in Britain. Other railroads soon followed. The policeman receiving a train would telegraph back to the previous station to alert them the block was now clear, and another train could be given the authority to move to the next block. Soon after, remote control of signaling apparatus was introduced, and with it came the wide adoption of the semaphore, an early mechanical signal made of a tilting blade with various colored lenses [1]. The semaphore was weighted so that if the controlling mechanism broke, it would fall to restrictive indication.

A semaphore signal at "danger" in Arbroath, Scotland [1].

Traffic on the railways was increasing dramatically, and higher speeds were demanded. Railroads needed a way to communicate to drivers in advance the instruction (called an indiction) of an upcoming signal, as higher running speeds meant that drivers could no longer stop within their range of vision. To warn a train of an upcoming restrictive signal, "distant" signals were placed at least one braking distance before their corresponding "main" signal. Upon seeing a restrictive distant signal, a driver would have enough time to bring their train to a stop before the actual main signal.