The principles of semaphore signalling as can
be seen around the United Kingdom today go back to the 1850s but at that
time there was no standardisation amongst signals for each railway company
had its own ideas. It wasn't until the 1920s that national policies were
applied, and the formation of the "big four" railway companies in 1923 is
a convenient place to regard as the point at which the signals in use
today became universally adopted. Of course, older signals were not wiped
out overnight and many survived (perhaps in modified or repainted form)
until recent years and a few can still be found around the railway system.
The earliest semaphore
signals were known as lower quadrant signals, because the arm was lowered
from horizontal to an inclined position when cleared. The first type
lowered the arm to a vertical position, concealed inside the signal post,
but this was abandoned in early years as the lack of a visible signal
isn't the best way to indicate "clear" - a problem that can be illustrated
by the behaviour of motorists when they find traffic lights unlit.
The other three members of
the "big four" were quick to adopt upper quadrant signals, so that gravity
could be used to return signals to danger. This kind of signal is now
almost universal in semaphore areas outside Network Rail outside the Great
Colour Light signalling is
now the accepted standard on Britain's railways and newly erected
semaphore signals are now rare. Sometimes colour light signals are
directly substituted for (and often mixed with) semaphore signals and the
equivalent indications are also shown on these pages. Although the
indications are similar, this use of colour light signalling is quite
different to true Multiple-Aspect Signalling which will be dealt with
All signals are defined as
on when showing a danger (horizontal) indication, and off
when showing a proceed (angled at 35 to 85 degrees from the horizontal)
indication. Signal posts may be of wood (painted white or creosoted),
steel (usually painted white or silver) or concrete. Signal arms are
painted wood or steel, or made of enamelled steel.
Signals are most commonly
found on the left-hand side of the line they apply to, although there are
At any signal box, there may be several
stop signals serving each line. Most commonly there are two, with the
first to be reached by a train (known as the home signal)
protecting any points, level crossings etc., and the second (the
starting signal) ahead of any points and guarding the entrance to the
block section ahead. More complex layouts can have many stop signals per
line, perhaps named Outer Home, Intermediate Home, Inner Home, Starting,
Advanced Starting, Outer Advanced Starting) whilst in smaller
circumstances just a single home signal may suffice.
Whichever of the signals allows entry into
the block section ahead is also termed the section signal, but this
is more of a technical term and is not normally used from an operating
point of view.
The colour light equivalent can be in two
forms. It will show red or green if there are no other stop signals ahead
worked from the same signal box (and as long as it isn't also acting as a
distant signal - explained later) but if there is another stop signal
ahead it will show yellow until all such signals are cleared.
Taking account of the description of Rule
39 above, it can be seen that if a driver passes a distant signal at on,
he will be prepared to find the stop signal ahead on, but if that signal
is off (without his brought nearly to a stand) he can expect all stop
signals ahead (worked by that box) to be off.
||Where it is
necessary to give a driver special information about the section ahead,
a subsidiary signal may be mounted below a stop arm. These consist of a
small red arm with horizontal white stripe. When off, a letter C, S or W
will be displayed, which indicate which of three functions apply:
- C - Calling-on signal. This indicates
that the section ahead is occupied, and only applies on permissive
block lines, station areas and other special locations.
- S - Shunt-ahead signal This indicates
that the signal may be passed only for shunting purposes, only by the
necessary distance to achieve such a shunt. Only provided on section
- W - Warning signal. This indicates
Section Clear but Station or Junction Blocked, an old-fashioned
term found in Regulation 5. Under normal block working conditions, an
additional ¼ mile safety margin is allowed when a train is accepted
into a block section, but at specially defined locations this
requirement is waived. This signal instructs the driver to proceed
through the block section with caution.
Signals used for shunting purposes are
distinct from stop signals as their function is different. They do not
indicate that the line ahead is clear, but that movements may proceed
as far as the line is clear.
Shunting signals are in the form of a
miniature semaphore arm, mounted at ground level or on a signal post. Many
of those at ground level have the arm superimposed on a white or black
disc to aid visibility. This type is generally called a disc signal.
semaphore shunting signals
combined home and distant signals
boxes are close together, the necessary position of a Distant signal
sometimes falls within the area of the adjacent box's signals. The
Distant signal can be mounted beneath the Section signal of the box in
rear, and it will be interlocked or slotted so as to ensure it is
only cleared if the Stop signal has been cleared.