1. The basic principles

On this page:
[ Principle | Block Section | Signals | In Advance or Rear | On or Off | Normal or Reverse ]


The principle of the British Railways Absolute Block system is quite straightforward. Only one train should be in any one block section at a time. The Absolute Block system only applies on double or multiple lines where trains always use each line in a pre-determined direction. Separate regulations apply to single and bi-directional lines.


Block Section

A block section is a section of line, stretching from the last stop signal controlled by one signal box to the first stop signal controlled by the next signal box. The terms first and last refer to the order they are seen by a driver as he travels along the line.



A typical signal box has one or more stop signals and a distant signal on each line it controls. These are described and illustrated in the British Railway Signals pages.

From a driver's point of view, the first signal he sees is the distant signal. This signal, if off, indicates that all signals worked from the next signal box ahead are also off. If it is on he needs to be prepared to stop at the next stop signal. Distant signals are always positioned a full braking distance (allowing for the maximum permitted train speeds and gradients) from the next stop signal.

The next signal a driver sees is the first stop signal controlled by that signal box. This is called a home signal. At more complex boxes there may be several home signals in succession. The last signal controlled by that box is the starting signal or starter. This controls the entrance to the next block section.

Here is a simple track layout at an imaginary signal box, which will be referred to as Box B. The box controls a double line of railway, with a cross-over road and exit points from a loop line. The uppermost line in the drawing is known as the Up Line, and trains travel from left to right along that line. The lower line is the Down line, for trains travelling from right to left.

Diagram of a simple signal box

The numbers on the diagram refer to the levers controlling the signals and points. Lever 1 works the Up Distant Signal, which is positioned a full braking distance from signal 2, the Up Home Signal (the first stop signal described above) and marks the exit of the block section from Box A.. These two signals are in rear of the box. There is no starting signal, and signal 2 also controls the entrance to the section ahead. In the Down direction, there are a Down Distant (22) and Down Home (21) - both in rear of the box, plus a Down Starting signal (20) in advance of the box that controls the entrance to the next block section. Signal 17 controls departure from the Loop onto the Main line. Signals 8, 10 and 15 are shunting signals for movements through the crossover points 9 or loop points 16 when reversed. Finally, signal 3 is a draw-ahead signal which permits trains to draw forward into the block section only as far as necessary to clear the crossover points (9) in order to cross to the Down line.


In advance of, or in rear of

A term used regularly is to talk of in advance of and in rear of. These always apply to the driver's viewpoint from his footplate. The next signal he reaches is in advance of him. The last signal box he passed is in rear of him.


On or off

In railway terms, a signal that is on is in the danger or caution position. It is off when it has been cleared for the passage of a train.


Normal or reverse

These terms specifically apply to lever positions in a signal box. When back in the frame, it is normal, and when pulled over it is reversed.