This page explains the signalling principles implemented in SigScribe4. They are based on British practice.


SigScribe4 - Documentation - The Principles of Railway Signalling


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The Principles of Railway Signalling

SigScribe4 is based upon typical British Railways practice but that does not limit its application to the many and varied systems throughout the world or to particular periods of history.

For the purposes of understanding how to apply SigScribe4 we don't need to talk too deeply about the methods used to regulate the flow of trains between one signalling location and another, but mainly to understand the information that a signal conveys to the driver of an approaching train.

Firstly, let's define some categories of signal.



Semaphore signals are those which use coloured and shaped arms which are usually mechanically operated by wires connected to levers in a Lever Frame. Their "normal aspect" is "danger" indicated by the arm being horizontal. Their "reverse aspect" is "clear" indicated by the arm being angled, in some cases upward at 45 or 90 degrees, or downward at 45 degrees. A signal may also be referred to as being "on" (showing its "normal" aspect), or "off" (showing its "reverse" aspect).

SigScribe4 incorporates four types of semaphore signal. Their depiction in SigScribe4 and their meaning is as follows.

 [Stop arm]A stop signal is understandably a signal at which a train may be required to stop. It is often loosely called a home signal although in strict terminology the term refers to a stop signal on the approach to a signalling location. At danger, it indicates that the driver must not allow a train to pass such a signal. At clear a train may proceed.

 [Distant arm]A distant signal arm gives an advance warning to a driver about the state of any stop signals ahead. At danger, a distant signal indicates that the approaching train must expect to stop ahead and brake accordingly. At clear, it indicates that a clear route is set past stop signals ahead. (In most railway systems a distant signal is interlocked with all subsequent stop signals. However in some "lower speed railways" it may be interlocked only with the first stop signal on the basis that the signalled area being approached is to be negotiated at a sufficiently low speed that advance warning of remaining signals is unnecessary. SigScribe4 can cater for both situations.)

 [Subsidiary arm]A subsidiary signal is a stop signal which, when clear, gives permission for a train to proceed at low speed into a section of track that may be occupied by another train. This could be, for example, for shunting purposes or to allow a second train to share part of an occupied platform. They are also typically used to control movements from a main running line into a siding or goods yard, or for movements from a siding or yard onto a main running line. Subsidiary signal arms take many forms and are usually smaller than standard stop arms.

 [Ground disc]A ground signal can take many forms including the disc signal depicted which has a red bar across a white disc. The disc face rotates 45 degrees to indicate clear. Miniature semaphore arms are also used. They are subsidiary signals and carry the same meaning as noted above. They are commonly referred to as shunt signals.

There are several ways in which semaphore signals indicate specific routes where more than one exist. Where the number of possibilities is reasonable there may be one arm per route. In more complex situations a route indicator may be used in conjunction with a single arm. The route indicator will show different numbers or letters to specify the route set. In multi-arm arrangements there are two styles used.

  1. Arms may be mounted on separate posts such that, for example, an arm mounted to the left of another indicates a route to the left. The heights at which arms are mounted is also used to indicate the relative importance of a route so that the main line would be indicated by the highest arm and so on. Although stop signals actually control the entry to particular routes, advanced indication of route can be provided by multiple distant arms arranged to reflect the corresponding stop signals ahead.
  2. Arms may be mounted in a vertical arrangement on a single post. In this case the higher the arm, the further left is the route indicated.

SigScribe4 allows for either separate arms to be specified or a numbered route indicator is also available.Semaphore RI


Colour Light Signals

Colour light signals can also be classified as distant, stop, or subsidiary. In British Railways practice signals may be 2-, 3- or 4-aspect, and subsidiary signals may be "position lights".

 [Stop head]A colour light stop signal is one capable of displaying a red aspect. The red aspect indicates that a train must not pass this signal.

 [Distant head] [O-distant head]A colour light distant signal is one which is not capable of displaying a red aspect. A single yellow (caution) aspect tells a driver either that the following signal is at red, or that a lower speed diversionary route is set at the following signal. The double yellow aspect is a preliminary caution indicating that the following signal is at caution. It is not uncommon for colour light distant signals to be used in mechanically signalled areas. As train operating speeds have increased on many railways, and consequently so have braking distances, colour light double yellow signals are sometimes used as "outer distants" in semaphore areas to give the required braking distance.

With all colour light signals green is the all clear or proceed aspect.

Unlike traffic lights, the red lens of a railway signal is at the bottom. It is also traditional to refer to the colour yellow rather than amber. With a 4-aspect signal the order of colours from the top is yellow (used for double yellow), green, yellow, red. The reason for the yellows being separated is to make the double yellow aspect more discernible at distance.

 [Subsidiary PL]Subsidiary signals have a similar function to their semaphore counterparts. They are typically of the "position light" variety where a clear indication is two white lights at a 45 degree angle. When additional to a main signal they show no light as their normal indication.

 [Ground PL]Ground signals may also be position lights, their normal (stop) aspect being a red light and a white light in line horizontally, or, more recently, two red lights in line horizontally. The clear aspect is two white lights at 45 degrees. Ground signals in colour light areas may also be miniature colour lights, or discs as described for semaphores.

 [Colour light RI]Route indication with colour light signals can be with multiple heads, one for each possible route, but this is uncommon. Another uncommon practice is to use a subsidiary yellow light to the side of the main head. Much more common is a display panel indicating by letter or number the route set, or a "feather" indicator in which a series of white lights in lines at 45 or 90 or 135 degrees either side of vertical represents the route set.


Interlocking & Levers

Quite early in railway history accidents forced the realisation that signals and points needed to be interlocked in such a way that contradictory indications were impossible. Mechanical ingenuity and later electrical circuits played an essential part in maintaining railway safety.

Interlocking is achieved in mechanical signalling by locking bars which engage with matching slots in tappets connected to the operating levers which are arranged in a Lever Frame. This Lever Frame and associated interlocking may be housed in a signal box (or cabin), or as a "ground frame" in smaller less frequently used locations. Additional interlocking may also be provided at pointwork or at signals. This article may be helpful in understanding the principle of mechanical interlocking.

Levers operate items such as points and signals through rods and wires, or electrical devices such as colour light signals are actuated by switches operated by the levers.

Levers are colour coded as an indication of their function. The following code is typical and is implemented in SigScribe4. Check this web page for a more definitive list.

 .. Black - points.

 .. Blue - facing point locks. ( What are facing point locks?)

 .. Black / Blue - points with combined facing point locks.

 .. Red - stop signal (including subsidiaries).

 .. Red / White - stop signal electrically released by "line clear" indication from the next signal box.

 .. Yellow - distant signal.

 .. Red / Yellow - intermediate block signals. (What are intermediate block signals?)

 .. Brown - level crossing gates lock.

 .... Black / White Chevrons - detonator placers for UP and DOWN lines respectively.

 .. White - spare lever.


Control of Signals & Points

Signals can be classified as:

  • Controlled. With mechanical signalling each signal is by nature controlled - it's state is directly controlled by the operation of the corresponding lever. Colour light signals can also be fully controlled by the operation of some type of interlocked switching mechanism.
  • Automatic. Automatic signals are controlled entirely by the passage of trains operating track circuits. Examples of automatic semaphore signalling exist but automatic operation is primarily associated with colour light schemes. In a typical example a line is divided into sections and a signal placed at the entry to each section. Immediately a train passes into a section, the associated signal changes to red so as to protect the train from the next train following. Once the train has cleared the section (plus a safety overlap distance) and is protected behind the next signal, the first signal will change to yellow. When two sections are clear a 3-aspect signal will change to green, or a 4-aspect to double yellow. A 4-aspect signal will show green when three or more sections ahead are clear, while a 3-aspect signal will show green when two or more sections ahead are clear.
  • Semi-automatic. These can operate either as fully automatic signals or they can be switched to a manual mode is which they will change to danger with the passage of trains but will only clear under operator control. Such signals are essential at stations and junctions where multiple routes exist.

The MODRATEC Lever Frame can be applied to controlled signals (semaphore or colour light) and to semi-automatic colour light signals in which case the lever for a signal effectively becomes the means to switch modes while maintaining full interlocking with other points and signals.

Points on main running lines can be operated mechanically by rods connected to levers in a Frame or by pneumatic or electric motors. An important feature is that facing points must be secured in the required state to avoid trains "splitting the points", an event that every model railway operator has surely witnessed. Facing point locks (FPLs) achieve this requirement. Sometimes the locking mechanism is operated in conjunction with the points changing device, or it can be operated and interlocked separately. In the model situation it would be very rare for a modeller to reproduce working FPLs, but there is no reason to avoid the inclusion of levers to operate imaginary FPLs and thereby add a further element of operational interest and accuracy.

Intermediate block signals deserve a mention because they can be implemented in SigScribe4. Railway safety is predominantly based on the block system in which lines are considered to be broken up into block sections in which no more than one train is permitted under normal circumstances. In the case of automatic signals described above each section between signals constitutes a block. In traditional mechanical signalling a block section normally extends from one signal box to the next. Obviously, the length of time required for a train to pass through a block section determines the capacity of a line in terms of trains per hour. This capacity can be increased (virtually doubled) by placing a set of signals midway through the section allowing a second train to enter the section once the preceding train has passed the intermediate signals and is protected by them. A stop signal and distant signal are used and operated from a single lever in the preceding signal box. Semaphore intermediate block signals would generally be too remote from the signal box for mechanical operation so that electrical or pneumatic control would be typical.


The Lever Frame

Lever Frames in the real world usually control train movements in a relatively small area partly because of visibility issues and partly because of practical distance limits on mechanical linkages. It was not uncommon in earlier times to find a signal box controlling each end of a moderate sized station, or a signal box at each corner of a triangular junction.

Levers in a Frame are identified by numbers corresponding to a mimic panel showing the track layout and signal locations relative to other relevant physical features. Levers are colour coded. The function of each lever is defined on its identification plate together with a list of any other levers that must be pulled in order to release the one identified.

The "normal" position for a lever is away from the operator. It is said to be "back". This position corresponds to the danger or "on" position for signals, the normal position for points, the free (unlocked) position for FPLs, the locked position for level crossing gates, and detonators off the track for detonator placers.

When a lever is pulled it is in its "reverse" position, or it is said to be "over".

Levers can be locked in either position by the interlocking system.

Some additional electrical interlocking is normal in the real world to further improve safety.



Slotting is an interlocking technique applied to certain signals to prevent a confusing or dangerous display.  One common use is where, on the same post, a stop signal is controlled from one signal box and a distant signal is controlled from the following box.  This happens where block sections are short.  Slotting prevents the distant signal from showing clear unless the stop signal is also clear.  SigScribe4 can simulate this action within the Lever Frame interlocking. Another situation in which slotting is applied is that where a single signal arm is controlled from more than one Lever Frame. Again, this can happen where sections are short and the location of one signal box's starting signal corresponds to the next signal box's home signal. Slotting is used to ensure that such a signal shows clear only when both boxes have pulled the corresponding levers. MODRATEC has developed a mechanical gate which can be used to 'slot' signals controlled from different locations. (See the 'WIT Products' page at our web site.)



Detonators may be used to give an audible warning of danger. These are placed, usually in pairs, on the rail and explode under the weight of the leading wheels of a train. They are used in conditions of poor visibility. While detonators can be placed by hand, they may also be placed by a mechanism linked to a non-interlocked lever in a Lever Frame. The mechanism may also co-act with a standard signal lever.


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The Basics of SigScribe4

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The Principles of Railway Signalling

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