Basics of German Railway Signalling

Early development

From the early 19th century onwards, the railway, as the engine of the industrial revolution, spread rapidly throughout Europe and also reached Germany in 1835. Here, the first train travelled between Nuremberg and Fürth, reaching top speeds of up to 50 km/h. Despite initial scepticism, the new means of transport quickly spread throughout the country. In particular, the high efficiency of the railway, achieved through the low friction of the wheel-rail system used, surpassed any other means of transport known at the time and even exceeded the car to this day.

Mechanical interlocking type "Einheit" at the railway school "Eisenbahnbetriebsfeld Gotha"

However, this system also created the biggest disadvantage of the railway: the extremely long braking distance of the locomotives and carriages, which can be up to 1000 metres for modern trains travelling at speeds up to 160 km/h. This makes it impossible to operate a railway line like a road at a visual distance. Initially, this problem was irrelevant, as only one train ran at a time on most railway lines anyway. The connection of the networks of individual private railways to form a nationwide rail network and the introduction of through express trains between metropolises brought this difficulty back to the fore.

The most obvious solution was the time interval regulation: trains were only allowed to leave a station towards the free track a prescribed time interval after the last train. It was only necessary to wait a few minutes after fast passenger trains, but a quarter to half of an hour after much slower freight trains. Not only was this regulation inefficient, but it also led to several railway accidents caused by inattentive railway staff or trains broken down on the track. So in the mid-1860s, work began again on a new solution to this problem.

Train operation with block signaling

In the 1860s, partly due to the invention of early railway signals, travelling with block signaling emerged as a solution to this problem. The system proved its worth, was introduced by law on all lines in 1907 and is still the most important guideline in German railway traffic today. It can be described as follows:

Railway lines are divided into different block sections by signals. Only one train at a time is allowed in each block section. If a train is travelling through a block section, the signal behind it must be set to 'stop'. It may only be switched back to 'go' when the train has left the block section and the following signal is set to 'stop'.

This can be illustrated graphically as follows:

Drawing of the block sections on a single-track line with direction of travel left to right. All signals are currently in the stop position

Now a train is to pass the first block section. All signals A, B, C are set to the 'go' position by the responsible signal boxes.

All section signals in go position. The train is approaching section signal A.

After the train has entered the first block section, signal A behind it is set to 'stop'. No other train can now follow it into the occupied block section.

The train has entered block section A, the related section signal is in stop position.

The first train is followed by a second, yellow train. Meanwhile, the first, red train has left the first block section and entered the second one - section signal B is falls into the 'stop' position. Section signal A can then be set back to the 'go' position.

Now, a second train appears ready to enter block section A. Meanwhile, the first train has left block section A and entered section B. Section signal A is now in go position, section signal B in stop position.

The yellow train now enters the first block section. The first block signal A then switches to the 'stop' position.

Block sections A and B are occupied by the yellow and red trains. Signals A and B are therefore in the stop position.

In the following, section signal B for the yellow train cannot be set to the 'go' position. Although the following block section is already free, signal C is not in the 'stop' position. The yellow train could pass signal C and would be in a block section with the red train.

Block sections A and C are occupied. Although block section B is already free, signal B cannot be set to 'go' because signal C is also still showing 'go'.

Once signal C has been brought to the 'stop' position, the yellow train can also finally section B.

Block section C is occupied, while the yellow train is entering block section B.

The train reporting procedure

At the beginning of the railway, the distances for signal operations were quite short, up to a maximum of 1000 m away from the signal box. As a result, most lines had many signal boxes only for operating the section signals, so-called block posts. Communication between these block posts and stations was necessary to ensure that there was really only one train in a block section at any one time. Initially, this took place via optical telegraphy or Morse code, but later via cable telephones. To avoid misunderstandings - with potentially fatal consequences - the train reporting procedure was introduced, which recognises standardised messages and wording for the most important operational events.

This article provides a rough overview of the most important train reporting messages. For a detailed explanation, please click (here).