This section of the wiki is designed to introduce users to the principles behind signalling. If you are new to SimSig this section will explain the different components which we simulate within the simulations.

This page is split up into various sections which each deal with a separate topic. Each topic has a short introductory paragraph. Followed by more detailed information.

Note : Please feel free to add to this section.

Please note this page is still under construction and is not yet complete.

See also Signal Aspects, Overlaps, Shunt Signals, Setting Routes, Types of Route.

Signals tell the driver what to do, and are contolled by the signaller. To clear a signal, the signalman must set a route from one signal to another. In SimSig, this is done by left clicking a signal (the 'entry' signal), and then left clicking the next signal along in the direction the train needs to be sent. Routes can be cancelled by right clicing the signal (depending on the options in the F3 list, you may have to select 'cancel' from a drop down menu. The track between one signal and the next is called a 'section' of track. There are 2 basic types of signal: Main and Shunt.

Main signals are shown by a coloured circle. These colours are called aspects, and are as follows:

Colour	SimSig Indication	Meaning
Red		Stop (with subsidiary)
Yellow		Caution, the next signal is showing a red aspect
Double Yellow		Preliminary caution, the next signal is displaying a single yellow aspect
Green		Clear, the next signal is displaying a proceed aspect
Flashing Yellow		This indicates a lower speed diversion ahead but not severe enough to require approach release. The next signal will be showing a steady single yellow aspect.
Double Flashing Yellow		This indicates a lower speed diversion ahead but not severe enough to require approach release. The next signal will be showing a flashing yellow aspect.
Flashing Green		These were used in the Peterborough - Grantham section to indicate 140mph running. Although still displayed, these have no special indication anymore and thus SimSig does not support them.

Note that some signals may not have a double yellow aspect, and that buffers are treated as a red signal. Routes can only be set from main signals to main signals, unless the main signal has a subsidiary signal (see below).

Some signals have an R next to them. This means they are repeater signals, and if the signal ahead is displaying a proceed aspect then they show a green aspect. If the signal is displaying a stop aspect then the repeater will display a single yellow aspect. Some repeater signals (marked RR) have an additional yellow light which allows them to show a double yellow aspect.

When a route is set to from a main signal to a main signal, not only must all the track between the first signal and the next be clear, but there must also be a length of track clear after the signal (the length is based on many factors), called the overlap. This is so that if the driver misjudges the braking approaching the signal, or is unable to stop at the signal for whatever reason, the train will not derail on any subsequent points or collide with any conflicting movements. Note that trains in SimSig will not pass signals at danger.

Also known as Position Light (PL) or, when situated at ground level, a Ground Position Light (GPL).

These signals are a quarter circle. Their aspects are as follows:

Colour	SimSig Indication	Meaning
Red		Stop
Yellow		Stop, unless the route is not set in the direction for which the signal can be cleared (see below)
White		Signal can be passed at caution towards the next stop signal or buffer stop

In real life, red is shown by two horizontal red lights (or in older installations, one red and one white, the red on the left), and white is shown by two white lights, one above and to the left of the other. The signals are usually on the ground, as opposed to being mounted on a post like main signals (though the latter is sometimes seen). Yellow is shown as with red but, obviously, using yellow lights (so either two yellows or a yellow and a white).

Shunt signals with yellow lights are sometimes found at sidings where there is need for trains to move along the sidings and into the headshunt, but without proceeding onto the running line. The yellow indication means “stop if the points are set for a movement that would take the train on to a running line”. Trains are permitted to pass the signal whilst it is displaying “ON” if the points are not set for the running line. The driver is expected to know which routes the signal has to be cleared for.

When a shunt signal is cleared (two white lights on the ground; a white triangle in SimSig) it permits the driver to pass the signal, and to proceed at danger being prepared to stop short of any obstruction. Unlike a main aspect, when a shunt signal is cleared it does not mean that the line is clear as far as the next signal.

Opposite route locking omitted: this where it is possible to set up and clear two opposing shunt signals to allow uncontrolled shunting moves within a yard.

Some shunt signals have LOS written next to them. This means that there is no route that can be set from the signal and no white aspect: it is there so that trains can back up for a short distance along a line that is not usually used in that direction, for example to access sidings or move from one platform at a station to another. In real life these “signals” used to be a signboard stating “Limit Of Shunt”, which is the meaning of the “LOS” caption; these signs are no longer standard, but lots still exist on the network. The modern replacement is a shunt signal with no white lights.

Sometimes, a route can be set from one main signal to another when there is a shunt signal in between. In this case, the shunt signal is known as a 'running shunt' or 'preset' and will clear, however the driver will know the aspect of the next main signal, and will not have to proceed slowly past each shunt signal as described above.

Generally, routes set from shunt signals do not require overlaps.

SimSig indication	Explanation
	A main signal with a subsidiary shunt signal attached

This is a signal that looks like a main signal, but has a grey quarter circle underneath the main signal circle. The grey quarter circle in real life is the two white lights of a shunt signal, which are unlit under normal circumstances and are ignored, however if a route is set from one signal to either a siding, or into a platform which is occupied (a 'call-on' route), or to a shunt signal, they will light and the driver will treat it as a shunt signal. Note that in the above situations the main signal will show red.

The paragraphs above deal with controlled signals, which require a route to be set for every train. In reality, most track is 'plain line' (away from points, certain types of level crossings, etc.), so automatic signals are used. Auto signals work themselves, with no action of the signaller, and display the relevent aspect based on whether the next section is occupied or not, and the aspect of the next signal. Some Automatic signals have a red button marked 'E'or 'R' next to them. This is so the signal can be set to danger (replaced) by the signaller in an emergency, or to protect certain level crossings (see separate topic).

A Semi-Automatic signal is an automatic signal that protects a ground frame or an emergency crossover. Releasing the ground frame will cause the signals to be replaced to danger (see separate topic).

Some controlled signals have a button that places them in automatic mode for a certain route, i.e. when a train passes it, it will re-set itself when the route has cleared again. These are used at junctions where one route is used more often than another, to save the signaller time. Don't forget to take the signal out of Automatic mode (by right clicking the button to make the circle hollow), before a train that requires another route approaches it. Note that taking the signal out of Automatic mode will not cancel the route set, until another train passes to clear the route. Some signals have more than one route that can be worked automatically.

Train detection is essentially 'how do we know whether a section of track is occupied by a train?' Train detection is normally accomplished using Track Circuits. How this works is quite technical, so for the best explanation, please see this Wikipedia Article

Sometimes, for very long sections, or one's where track circuits are unreliable (for example in the Severn Tunnel), Axle Counters are used. These essentially count the number of wheels entering a section, and the number leaving a section. If these values are equal, the section is assumed clear. Note that in SimSig, Track Circuit is used as a blanket term, since this is the most common method.

If a section is occupied, the track is red instead of grey. To show section boundaries, in the F3 options menu, tick 'show track circuit breaks'. Note that a thick grey vertical line instead of the usual short gap shows the end of a signal's overlap. If a track circuit is occupied, you can not:

• Change any points covered by the circuit.
• Set another route over the track circuit, unless it is identical to the previous route.
• Raise any Controlled Level Crossing covered by the circuit.
• Any occupied track circuit in a route will set the entrance signal to danger (unless the route is a shunt route at a location where call on routes are allowed.

If a track circuit has a route set over it, it is coloured white instead of grey, and will have a similar effect to an occupied track circuit, with the exception that if the route is part of an overlap , points can be changed if desired.

This is where the signal will replace to danger automatically on the release of the track circuit in the rear of the signal.

This is where the signal will replace on the occupation of the first track circuit in advance of the signal.

Level crossings are where a road or footpath crosses a railway line on the level.

Please see this page.

See also: Main ARS page

ARS is a feature included in many IECCs, which allows the signaller to hand over most control of an area to a computer, which regulates trains accordingly. The control area is split into several 'sub areas', which can be enabled or disabled as required. ARS also changes Train Describers when the train reaches it's destination (ACI). Please note that ARS IS NOT PERFECT. It has several limitations, the most important of which are:

• It will route trains by what is in their timetable. Because it is computer-controlled, it will not show the imagination of a real person and change a train's platform or line in the event of a failure- it will happily leave a train sat at a signal until the platform becomes clear.
• It is not allowed to set call-on routes, or routes into non-track circuited track.
• It cannot set routes into sidings unless a Slot control is provided.

Full ARS is provided on the following Sims:

• Peterborough
• Waterloo
• Liverpool Street
• Drain
• Edinburgh
• Swindon A&B (SwinDid)

One type of basic ARS routes trains on a First-Come First-Serve basis (FCFS), i.e. when a train reaches a certain point all the routes it requires are set through the sub-area. FCFS does not regulate. It is used on less-busy areas of the panel, or areas that are very routine, to allow the Signaller to focus his attention on other areas.

Another type of basic ARS is Main Line route setting. This will re-set routes on the main as soon as they are cleared within the sub-area. For example, Cambridge contains a sub-area that will re-set routes continuously through Royston.

To see where basic ARS is provided, see individual Sim manuals.

Automatic Code Insertion is provided on some Sims at certain locations, again please see the relevant Sim manual.

This topic covers how railway staff communicate with signallers.

This topic covers information relating to Track Circuit Block control.

See also As used in SimSig

A very detailed (and accessible) guide to Absolute Block working is available at The Signal Box

This topic covers information on controlling single lines. Including Tokens, and other methods.