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Signal Operation & Control

The following descriptions, words and diagrams have been put together by Phil Wimbush and were part of a talk he gave to the Ickenham & District Society of Model Engineers to explain how signalling works and are reproduced with his permission.

London Underground Signalling

The Requirement for Signals.

In order to provide a safe system of railway operation, Signals are used to control the movement of trains.

Safety is based upon the detection of trains to ensure they are kept a braking distance apart.

To Ensure this happens, electrical circuitry is installed in the running rails and connected to a colour light signal at the start of the section.

With a train in the signalling section the signal will be at Danger,when the train leaves the section, the signal will change to a Proceed aspect then the next train can proceed through the signalling section.

A Braking Distance?

The Whole signalling system is designed to keep trains a braking distance apart.

The Braking Distance is defined as the distance between the location where the brakes are applied, to the place where the train comes to rest.


The Braking Distance is calculated for each individual signal section based upon the following factors :-

Speed of the fastest train,

Weight of the heaviest,


Braking Capacity of the trains

Braking Capacity

London Underground trains have a very high brake rate compared to national railway trains,  so on LU trains can be brought to a stop very quickly, so trains can safely be signalled closer together than say a long heavy freight train, or a High Speed Train which needs more room to slow down and stop.

So on metro/LU lines signals are closer together. On the most intensely worked " main lines " signals are generally at least ¼ mile apart.

On an average LUL line, trains can get within 100yds of each other in total safety under normal signalling.

The Track Circuit

New York Subway invented the first track circuit in 1890's

First Large scale use in UK was on the London Underground from 1904 on Great Northern, Piccadilly and Brompton line. ( Piccadilly today ) which opened in 1906.

Today each Track Circuit is given an identifying number.

Basic Track Circuit - Clear

LU Track Circuits around 10v AC, Signal Aspects 100v AC

In this diagram, no train is in the Tack Circuit (or Block). The signal is Green - a Proceed aspect.  Note the small red joints at the start and finish of the block - these are Insulated Block Joints which separate one section from another.

Basic Track Circuit - Danger

A train has now entered the block - note the leading wheelset of the train.  Our signal has now gone to Red - a Danger aspect.

Insulated Block Joints

The Insulated Block Joints are visible in Line with the raised trainstop in this picture.

For the purpose of this explanation let us look at a simple, imaginary signal cabin. So.......

Welcome to Oak Signal Box

We are going to look at a small section of railway with modern signals controlled from a remote location, or even a computer based signalling system.

Note :-

Colour Light Signals

Track Circuits with reference numbers

Points and their Numbers

Train Describers

Automatic Signals are those with no points ahead which automatically clear to green when the section is clear.

Controlled Signals need to be cleared for each train.

Oak Signal Box Diagram

Train 7752 Arrives in Platform

Signal C25 is cleared to Green

Train 7752 Departs

Signal C25 Returns to Danger and track circuit 105 drops (becomes occupied, thus holding Signal C25 at Danger - this is the overlap for signal C25)

Train Proceeds ….

Train Clears Platform and overlap

Note train description changed for following train 6460 which would now have a clear signal into Oak platform

Train 6460 now in Platform

This train terminates at OAK so all passengers will be "detrained" onto the platform.

For C23 to clear….

Next Move…

Train 6460 is now signalled to enter the reversing siding. Note all four ends of points reverse and shunt signal C23 clear.

Up Train 6458

An up train 6458 has now arrived. It is being held outside at Signal C20; this signal cannot be cleared because train 6460 has the route set across into the siding.


If, now train 6458 was to overrun signal C20 at Danger, assuming this is an LU line, the trainstop by the signal would strike the Tripcock on the train and the train would be stopped, however, train 6458 may drop 104 Track, which needs to be " up " to allow Signal 23 to be cleared for train 6460 so signal 23 would return to Danger, the trainstop would raise and if the 6460 Train driver does not see it in time, he too would become Tripped and be stopped. So, both trains Would be stopped automatically by the safety signalling system.

Train 6458 has passed signal C20 at Danger, misjudged braking and just slid past by a few feet on greasy rails !!  So - the result is..............


Train 6458 Has passed C20 at Danger, dropped 104 track and signal C23 has returned to Danger.

Route will stay set and locked for 6460 to Siding.

It is decided to move 6458 back inside signal C20.

6458 Back inside signal C20

Normal Working Resumed, Signal C23 cleared again.

Train 6460 Departs Platform

Note signal C23 Returned to Danger when 105 track drops

Still Moving….

Train 6460 now entering siding, rear of train has cleared & released No.76 Crossover which has normalised, and down trains can now run, Up Train 6458 Still waiting.

All Clear !

Train 6460 now fully berthed in siding and both lines are nowclear for through trains, note both trains with green signalsand 75 + 76 Crossovers now in the " normal " position.

What Is A Signal Failure ?

Remember when we were looking at OAK and what had to happen to clear signal C23, If any one of those conditions were not met, then the signal would not have cleared.

  • All four ends of the crossover must be proved and locked reverse.
  • All five track circuits need to be showing " up \ clear "
  • The Trainstops need to be proved in the positions required.

Any one of those checks could fail for any one of a huge number of reasons.

In a real life area there are other considerations, such as Emergency Stop Plungers, Staff Key Protection Switches and shunters plungers to check too !

Any one of the above could cause the signal to remain at danger.

So the " signalman " will go through his checks, looking at all of the options until the defect was found.

What does a Green Signal tell a train driver?

" Section Ahead Clear Proceed Normal Speed, Points (if any) are set and locked "

So back at OAK if we cannot clear signal 25 to proceed how can we move the train ???

Because we cannot clear signal 25, we cannot guarantee to the train driver what the points (76a) are doing or what position they are in, or tell him anything about the state of the road ahead.

He's going to have to pass signal 25 at danger, but what has to happen first ?

The points 76a would need to be secured or clamped manually in the normal position.

Track Circuit Failure

Track 103 is showing occupied, there is no train on that track. The Signalman is unable to clear signal 25. For train 7752 to Proceed on the down line, the points ahead of train 76a need to be secured in the normal position, then the train can be Authorised to pass signal 25 at DANGER. This securing of the Route is what takes time when a failure is discovered.

Scotch Block

The scotch block ( pictured ) in place in between open blades. A Clip (basically a specially built G Clamp ), goes underneath rail and Clamps closed blades shut. It can be Padlocked if required.

This needs to be done to all Points On a route where a signal is failing.

What happens then ?

Train driver is given permission to pass the signal at Danger

Makes a Announcement

Closes his doors

Sounds his whistle

Moves slowly forward

Train will become tripped on the trainstop and will STOP

Driver will reset his Tripcock, check his doors are still closed

And when brakes have released…move off.

Train  now has an automatic speed limit for 3 minutes.

This is called SCAT ( Speed Control After Tripping )

Motors will automatically shut off if train goes above 10 MPH within those first three minutes of being tripped.

After passing two signals at green, train can revert to normal.


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