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Railway Signaling In United Kingdom

The railway signaling system used across the majority of UK rail network uses lineside signals that control the movement and speed of the trains.

The most modern-day system uses two, three, and four aspects of color lights using track circuit block signaling. It's a development of the original absolute block signaling and still being used on many secondary lines. Then the use of lineside railway signals in Britain is restricted with a maximum speed limit of 125 miles per hour(201 km/h), this is the maximum speed limit at which the train can travel safely by using the line-side signaling. As the train runs faster, it is not possible for the train driver to safely read the color light signaling. If the trains operating at a speed faster than 125mph usually use an in-cab signaling system that automatically determines and calculates the speed restrictions.

In early British Railways, "policemen" were employed by every railway company, their jobs were many & varied. One of their key roles was giving hand signals to inform the train drivers as to the state of the line ahead. At this time they had no communication with their colleagues along the line, and the trains were only protected by a time interval. After a train had passed him the policeman would stop any following train if it arrived within 5 minutes, for any between 5 and 10 minutes after, then he would show a caution signal after 10 minutes, then this line was assumed to be clear.

If the train speeds increased, it+ become difficult for the engineman to see the hand signals given by the policemen, at this moment the railway provided various types of fixed signals to do the job, operated by policemen(signalmen).

Running Signals in Railways

The terms "on" & "off" were used to describe the British Railway Signals. As we describing an older semaphore, "on" refers to a signal arm in the horizontal position and "off" means the signal raised upwards or lowered down from the pivot point(up to 60°). With regard to the newer color light signals "on" is synonymous with a danger signal and all other aspects are considered to be off. A good way to remember this is to refer to the state of the red light. If it is lit, then the signal is "on", if the red light is unlit, then the signal is "off".

Colour Light Signals

In the UK, the color light signals display seven aspects

1. Green - Clear The train may proceed subject to any speed restrictions applying to the section of the line or the train itself.

2. Double Yellow - Preliminary Caution

The next signal displaying a single yellow aspect

3. Flashing Double Yellow

This indicates that the next signal is showing flashing yellow 4. Single Yellow - Caution

At this time the driver must prepare to stop the train at the next signaling 5. Flashing Yellow

Warns that the lower speed diverging route is set, at a high-speed turnout 6. Red - Stop/Danger

7. Flashing Green

The train may proceed at line speed during this time. Commonly green aspects and the four yellow aspects are known as "proceed aspects" as they allow the pass the signal. And the red signal indicates that the train to stop.

The two aspect systems use red & green only, three aspect systems include yellow. Then the Four aspect signaling also double yellow and which is the most used on busy routes to allow shorter headways and fast routes to offer longer braking distances.

Unusual Colour Light aspects

1. Flashing Green - Flashing green aspects were employed on the East Coast The Main Line north of Peterborough. Installed for 140 mph(225 km/h) and running a connection with the testing of the new InterCity 225 electric trains, with a steady green limiting test trains with the normal speed limit of 125 mph(200 km/h). 2. Splitting Distance - In some locations approaching a junction two heads are placed side by side. When this signal (or the junction signal) is in danger, one head is dark and the other shows red or single yellow. The junction signal is not a danger, both heads show an aspect: one for route set ahead of the junction which shows the correct aspect while and others show single yellow. 3. Green over yellow, green over green - Liverpool Loop Line and London underground uses separate red/green "stop" and green/yellow "repeater" signals. If the repeater signal is at the same location as the stop signal, then it is placed underneath it and lit only until the stop signal is green. Thus the order of the heads ( top from bottom) Green, Red, Green, Yellow, and the aspects are Red, Green over Yellow, and Green over Green. 4. Yellow over green - Used in experimental "speed signaling" at Mirfield that offers an additional caution. It meant that the next signal was showing double yellow.

Read more railway signaling information at :https://www.sigtechrail.co.uk/

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