Initially, all railways were single-tracked. Trains ran from both directions and met at the stations. To ensure it was safe to dispatch a train onto the line, the rule was early introduced that only one train could be between two stations at a time. With signals on the railway, it became possible to provide information directly to the train driver.

The safety of the railway was enabled from the beginning by telegraph contact between the stations. When the line was clear, the signalman could give the departure signal for the train. After that, there was no way to contact the train driver.

To increase safety on the railway, entrance signals were introduced at stations during the last decades of the 1800s. These so-called semaphores indicated to the train driver with arms whether it was clear to enter the station or if the train should stop outside the station area and wait for a clear signal.

For night operation, traffic was controlled with light signals: green for go and red for stop. Later, light signals were introduced for traffic in both daylight and darkness. The regular signals, called main signals, were also supplemented with pre-signals so that the train driver would know in advance what would be indicated further along the line.

Over time, railway signals have become more advanced and are part of the railway's increasingly comprehensive safety system. Initially, all railways were single-tracked. Trains ran from both directions and met at the stations. To ensure it was safe to dispatch a train onto the line, the rule was early introduced that only one train could be between two stations at a time. With signals on the railway, it became possible to provide information directly to the train driver.