British railway signals
Modern British signalling is based on a two, three and four aspect colour light system using non-permissive block rules. It is a basic progression of the original semaphore signalling that can still be found on many secondary lines. Colour light signalling is a reliable and safe system but has been superseded in many countries by the use of in-cab signalling. This is especially true on many high-speed railways such as those in France and Germany, which use the TVM and LZB cab signalling systems, respectively. At speeds above 200 km/h (125 mph), it can become difficult for a train's driver to vigilantly see each signal and react accordingly within the available sighting distance. This situation is exacerbated during periods of poor visibility during bad weather which is quite frequent in Britain during the winter months. The use of lineside signals is therefore normally restricted to railways with a maximum permissible speed of up to 125 mph (201 km/h).
Basic signal aspects
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Clear (Green) - The track ahead is clear. The train is permitted to travel at any speed up to the current line limit.
If the signal is a 4-aspect signal, then the following signal will be showing clear (green) or preliminary caution (double yellow). If the signal is a 3-aspect or 2-aspect signal, then the following signal will be showing clear (green), or caution (yellow).
Preliminary caution (double yellow) -
Be prepared to find the following signal showing caution (yellow).
Caution (yellow) -
The following signal will be showing Stop (red), and the driver must be prepared to stop at the following signal if it does not change to a less restrictive aspect before (s)he reaches it.
Stop (red)
Also known as 'Danger'. The driver must stop at the signal. The train may only proceed when the signal changes to a less restrictive aspect.
The red stop signal is non-permissive and must not be passed. This signal denotes that the block ahead is occupied and must not be entered for safety reasons. Additionally, a red signal will be shown on the approach to a set of points when a clear path through them is not set.
Junction signals
British signalling is unusual in that when a train approaches a junction and is taking a diverging route, the preceding signal will indicate the diverging route to the driver. A fixed lineside sign shows the speed that applies through the divergence. Most signalling systems around the world adopt the "speed signaling" philosophy where the safe route speed is shown with no indication of the divergence at a junction.
Signals which display whether the route is diverging from the route ahead will have a Route Indicator (colloquially called a “Feather” from its supposed resemblance to a popular inn sign) of white lights attached to the main signal.
If the feather is lit, then the train will be diverging from the main route at the next junction. A feather can point to the left or to the right, as appropriate.
A signal has more than one feather where there is more than one diverging route ahead. The lit feather indicates which diverging line the train will be directed to. In the example shown, there are two routes diverging to the left of the main line, and the train will take the first one of the two.
In areas where speeds are lower, and there are a number of routes which can be taken, “Alphanumeric Route Indicators” are used to display a number or a letter to denote the route the train is to take (e.g. a platform number or line designation). They may located above or beside the relevant signal. When a route is set and the signal is cleared, the relevant letter or number is shown.
On shunting signals, where speeds are much lower, a miniature version of the alphanumeric route indicator is used.
Speed restriction signs
Speed restrictions are imposed on a route to ensure a train is always travelling at a safe speed for the prevailing conditions along the track. Speed restrictions are imposed where the route encounters a hazard such as a tight radius curve, level crossings, certain junctions, tunnels and bridges and where the train is entering a terminus station.
Warning boards
Warning of a permanent speed reduction (PSR) ahead of 75 mph (121 km/h). Typically placed about a half mile to a mile before the start of the permanent speed restriction, depending upon the line speed and the speed reduction.
Warning of a permanent speed reduction on the left diverging route of 75 mph (121 km/h). Again, typically placed about a half mile to a mile before the permanent speed restriction on the diverging route, depending upon the line speed and the speed reduction.
Permanent speed restriction signs
Start of permanent speed restriction of 125 mph (201 km/h) on the main route
Start of differential permanent speed restriction, with two varying restrictions for different types of trains. The top limit is for non passenger carrying trains such as freight and the bottom limit is for passenger carrying trains. In this example, freight is limited to 35 mph while passenger trains are allowed up to the speed of 70 mph.
Start of permanent speed restriction of 40 mph (60 km/h) on the diverging route to the left. Placed before the diverging route to instruct or remind the driver of line speed on the diverging route
Temporary speed restrictions
Due to engineering works, a temporary speed reduction (TSR) may be enforced at a particular location.
Warning of temporary speed restriction of 40 mph (60 km/h) ahead
The start and termination indicators of a temporary speed restriction of 40 mph (60 km/h).
Repeater signals
Banner repeater signals are used to repeat the indication of the following signal, where the driver's view of the actual signal may be restricted e.g. by a bridge.
Banner Off – The signal being repeated is showing a proceed aspect, and so can be passed.
Banner On – The signal being repeated is showing a Stop aspect, and so the train must be prepared to stop at that signal.
"Off" Indicator - Like the banner repeater above, this means the associated signal is showing a 'proceed' aspect. These are mainly used at stations, for the benefit of the guard or platform staff. When the display is blank, it means that the associated signal is set to stop.
In addition, similar indicators at station platforms may display "CD" (Close Doors) or "RA" (Right Away).
Shunting signals
Shunting signals allow a train to move forward onto a line which may already be occupied. Shunting signals may be attached to a main signal in which case they are only illuminated when the shunting movement is required (known as a subsidiary signal), and they display two white lights at an angle of 45 degrees. The driver may pass the signal with caution at a speed which allows the train to stop short of any obstruction.
These signals may also be placed on the ground called a "ground position light" (GPL), or on a gantry with no corresponding main signal. In this case, the signals will show either two red lights or a red and white light in a horizontal arrangement, when no movement is signalled.
If two red lights or one red one white is shown, the signal must not be passed.
When a shunting movement is signalled, the signal will show two white lights in a diagonal arrangement. This means the driver may pass the signal with caution at a speed which allows the train to stop at any obstruction.
This type of signal is also used to designate a "limit of shunt" – the point up to which trains that are shunting are allowed to proceed. In this case, two red lights are displayed side by side, but no other aspect can be shown. No train is allowed to pass this fixed signal in the direction shown (this will be against the normal direction of travel on the track in question).
Here the two yellow lights or one yellow one white indicate a shunting movement is permitted past the signal but only for a move in a direction for which the signal cannot be cleared (for example, towards a headshunt rather than on to the main line). Again, two white lights at a 45 degree angle indicate shunting is permitted.
Junction signalling
When a route is set at a junction that involves the train taking a diverging route that must be passed at less than the mainline speed, a system known as approach release is used. There are a number of different types of approach release that are used on British railways but the most often used is "approach release from red". This system has the signal before the diverging junction held at red until the train approaches it, whereupon it changes to a less restrictive aspect with the appropriate direction "feather" of five white lights. This is required so that the signals approaching show the correct caution aspects, slowing the train down for the junction.
While the junction signal is held at red, the preceding signal will be displaying caution (yellow), and the one before that will display preliminary caution (double yellow) if it is a 4-aspect signal. This system allows for a gradual decrease in speed until a safe speed is reached for the train to move through the junction.
Another common system is "approach release from yellow with flashing aspects in rear". It is essentially similar to approach release from red, except that the junction signal is released from yellow and the signals in rear will flash to warn the driver that the train will be taking a diverging route ahead.
Where the turnout speed is the same as the mainline speed, approach release is not necessary.
On modern high speed routes such as the East and West Coast Main Lines, some turnouts at major junctions are designed to operate at maximum or near maximum linespeeds to keep the average speed of the journey as high as possible and reduce journey times as well as unnecessary wear on the train wheels, brakes and the track. Movable frog switches are occasionally employed to allow high speed running through the junction.
Unusual signal aspects
Flashing green
During testing of the Class 91 electric locomotives following the electrification of the East Coast Main Line in the late 1980s, British Rail introduced a flashing green aspect as authority to exceed 125 mph (up to a maximum of 140 mph). This does not apply to trains running in normal service however.
Meaning: The track ahead is clear and available for running in excess of 125 mph (201 km/h). The train is permitted to travel at a speed in excess of 125 mph (201 km/h), up to the current line limit (which may be as high as 140 mph). The following signal will be showing 'clear' (green or flashing green).
References
EuropeanBahn signalling guide to the East Coast Main Line Express in Microsoft Train Simulator
See also
- UK railway signalling
- Interlocking
- Railway signal
- Railway signalling
- Signal box
- Pass of Brander Stone Signals
- Signal passed at danger (SPAD)