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The basic, two-aspect, red/green signal is fine for lower speed operation but for anything over about 30 mph / 50 km/h the driver of a train needs a warning of a red signal ahead to give him room to stop. In the UK, this led to the idea of caution signals (originally called "distant" signals when they were mechanically operated semaphore arms) placed far enough back from the signal protecting the entrance to the block to give the driver a warning and a safe braking distance in which to stop. When this was developed for track circuited signalling, the caution signal was provided a block further back from the stop signal. Each signal would now show a red, yellow or green aspect - a multi-aspect signal.
| 2 Aspect Red / Green stop signal and auxiliary Shunt signal This signal was formally at Horsham Station in Sussex it is one of the earlier Westinghouse signals with 'Pigs Ear's side light signal indicator lamps to help train drivers see the signal aspect when they stopped close to a signal. Photograph © Andrew Babbs. |
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3 Aspect Red / Yellow / Green signal Again at Dorking station we have the three aspect Up Home signal located on a gantry to allow for better signal sighting for the train driver. There is also a raised shunt signal lower down the gantery for access to sidings. Tthis is another early Westinghouse signals with 'Pigs Ear's side light signal indicator lamps to help train drivers see the signal aspect when they stopped close to a signal. Picture reproduced from©Mark Adlington collection |
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Dorking Station Up Platform starter A close up picture of the 3 aspect starter signal located on the platform. One can just about make out the red aspect is alight, it also shows that the pigs ears can be fitted to either side of the signal as the blanking plates are clearly visible on this side. Picture reproduced from©Mark Adlington collection |
The multi-aspect signalling commonly used in the UK today is a 4-aspect system. It works similarly to the 3-aspect system except that two warnings are provided before a red signal, a double yellow and a single yellow. This has two purposes. First, it provides early warnings of a red signal for higher speed trains or it can allow better track occupancy by shortening the length of the blocks. The high speed trains have advanced warning of red signals while the slower speed trains can run closer together at 50 km/h or so under "double yellows".
This diagram shows four-aspect signals with (in the upper diagram) a high speed train with three clear blocks ahead of it and (lower diagram) a slower train with two clear blocks ahead of it. The lower speed trains can run closer together so more trains can be operated over a given section of line.
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Red Danger stop this at signal |
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Yellow Caution next signal set at Red |
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Double Yellow Next signal at Caution |
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Green Next signal at Double yellow or green |
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Junction Route Lights 5 white route indicator lights are mounted above the signal to indicate a left hand or right hand turn. Under these circumstance one set of 5 white lights are alight with the yellow, double yellow or green aspects. No white lights are used when the straight route is set. |
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Theatre Route Junction indicator Where 3 or more junction route indicators are needed there is a risk of confusion, therefore in these instances the Theatre Route Junction are used. In this example the letter 'U' would mean 'UP MAIN' another example would be 'S' for slow. It can also show 0-9 if needed for platform numbers.. |
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All 4 aspect photographs from the free photo website
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Banner or Repeater Signal
Picture courtsey of © Paul Vincent. |
Unusual signal aspects
There are some more unusual signal aspects in use today..
Flashing yellow -
a flashing single or double yellow indicates that a train is to take a diverging route ahead with a lower line speed than the main route, indicating to the driver to slow the train down in time for the speed limit of the diverging route. A flashing double yellow means that the next signal is showing flashing single yellow. A flashing single yellow means that the next signal at the junction is showing (steady) single yellow with an indication for a diverging route, and the signal after (in advance of) the junction may be red. When the train has neared the junction and slowed down, the junction signal will 'step up' to the correct aspect depending on the state of the line ahead.
Flashing green -
flashing green aspects are employed on the East Coast Main Line north of Peterborough. They were installed for 140 mph (225 km/h) running in connection with the testing of the new "225" electric trains, with a steady green limiting test trains to the normal speed limit of 125 mph. They no longer have official meaning, but remain in place and there are a couple of locations where the presence or absence of flashing provides useful information to drivers.
Splitting distants -
at some locations approaching a junction two heads are placed side by side. When this signal or the junction signal is at danger, one head is dark and the other shows red or single yellow. When the junction signal is not at danger, both heads show an aspect: the one for the route set ahead of the junction (left or right) shows the correct aspect while the other shows single yellow (or double yellow at an "outer splitting distant").
Green over yellow, or green over green -
the London Underground uses separate red/green "stop" and yellow/green "repeater" signals. If a repeater signal is at the same location as a stop signal, it is placed underneath it and lit only when the stop signal is green. Thus the order of the heads is (from top to bottom) green, red, green, yellow, and aspects are red, green over yellow, and green over green.
Yellow over green - this was used in the experimental "speed signalling" at Mirfield, abolished in 1970, to provide an additional caution. It meant that the next signal was showing double yellow.
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