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.
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
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.
Given that a each block
section involves two signalmen, it is necessary to have a strict
discipline in the way trains are admitted to the section to avoid
collisions. This is achieved with Block Instruments.
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
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.
keeping the trains apart — Regulation 4
Given that a each block section involves two signalmen, it is necessary to
have a strict discipline in the way trains are admitted to the section to
avoid collisions. This is achieved with Block Instruments.
Illustrated above are a
pair of block instruments that would control a double line piece of
railway between two signal boxes. At the top of each instrument are two
electrically controlled needles (D&E), capable of pointing at Line Blocked
(Line Closed or Normal on some instruments), Line Clear and Train on Line.
Beneath is a commutator handle (C) that allows the signalman to choose the
position of the lower of the two needles. At the bottom is a bell tapper
(A) - like a morse key - and a bell (B). The bell tapper rings the
bell at the next box. The commutator controls the lower needle in this
instrument, and the upper needle at the box at the other end of the
section. Some older instruments do not have all of the equipment built
into one unit; it is not unusual to find each needle, and the bell, housed
in separate units on the block shelf.
Different types of instrument exist on some lines, such as Lock and Block,
or two-position instruments. Whilst the basic principle of absolute block
remains, the method of working the instrument is different, and will be
detailed in future pages of this section.
The clearing point is a quarter of a mile ahead of the first stop signal
controlled by a box, and is a safety zone - an area that must not be
occupied or fouled by another train. It allows for the approaching train
being unable to stop at the signal, perhaps through slippery rails or weak
A tail lamp is a red lamp provided on the rear of all trains, and is a
fundamental part of the absolute block system. It is the only proof the
signalman has that the whole train has left the section.
If the thought of part of a train being left in the section seems
unlikely, you need to think again. Consider the fact that in 1960, many
freight trains had no automatic brakes other than those on the locomotive
so a driver could be unaware that his train had become divided.