by John Hinson

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 Western Zone.

Lower quadrant


Upper quadrant

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 separately.

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 exceptions.

stop signals

A stop signal has a red arm with vertical white stripe towards the left-hand end. It shows a red light when on and a green light when off. Trains must not pass stop signals when on except under specially authorised conditions - this is one of the most contentious issues of modern-day railway operation.

Note - in certain circumstances, the Colour Light equivalent may show red, yellow and green indications.

Lower quadrant Somersault Upper quadrant   Colour Light

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.

distant signals

A distant signal has a yellow arm with a vee-notch at the left end of the arm. A black vee-stripe is painted towards the left end. Distant signals show a yellow light when on and a green light when off.

A distant signal is the first signal a driver sees when approaching a signal box, and when off it indicates that ALL stop signals controlled from the box ahead are off. If on, a driver must expect to stop at the first stop signal. By definition, therefore, a distant signal has to be located a full braking distance from that stop signal, taking into account gradients and maximum permitted speeds.

Lower quadrant Somersault Upper quadrant   Colour Light

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.

subsidiary signals

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:
  1. 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.
  2. 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 signals
  3. 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 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.

Lower quadrant Upper quadrant   Colour Light


shunting signals

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

Sometimes, the view of a signal can be improved by bracketing it out from the signal post.

This method is often used where the line is curved, and also where there isn't room to position the signal alongside the line to which it applies.

Signals such as these are sometimes used in place of Stop signals on low-speed goods lines. This gives the same authority to enter the section as a calling-on signal would.

Lower quadrant Somersault Upper quadrant   Colour Light


ground disc signals

Ground signals are mounted at ground level - you'd never have guessed!

Their meaning is exactly the same as the miniature-arm signals above.

Lower quadrant Upper quadrant   Colour Light


yellow shunting signals

  Shunting signals with yellow arms (with, sometimes, a black band) are usually found at the outlet of sidings where there is also a headshunt.

Signals of this type should not be confused with Distant signals, as their purpose is quite different. The function is similar to that of the red arm shunting signal, with one additional feature. It may be passed when on for shunting movements along the headshunt. This saves frequent operation of the signal when shunting is taking place.

Lower quadrant Upper quadrant   Colour Light


combined home and distant signals

Where signal 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.
Lower quadrant Somersault Upper quadrant   Colour Light


outer distant signals

If the above arrangement does not provide adequate braking distance, an additional Distant arm can be placed on the previous Stop signal too. This would be called an Outer Distant whilst the other signal would be called the Inner Distant signal. Such signals require additional slotting to ensure that the Distant arm does not clear until the Stop signal ahead (aswell as the arm on the same post) is off.

The two signals in these illustrations represent the home and starting signals carrying the distant arms for the box in advance.

In extreme situations, the Distant signal of the previous box can be shared by both boxes, slotted so that it doesn't show off until both signalmen have operated the signal lever. This arrangement isn't totally satisfactory as drivers cannot tell (when the signal is on) whether to brake for the first or second box's home signal. This is the only circumstance in the principles of semaphore signalling where greater information can be given to a driver by a Colour Light signal.

The signals in these illustrations represent the distant, home and starter of one box. The distant arms below the home and starter are controlled from the next box, but the distant in the foreground is controlled by both boxes.

Semaphore   Colour Light