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The Development of Modern Signalling on London Underground
A history of the development of
signalling on London Underground since the days of electrification at the beginning of the
20th century.
Contents
Before Electrification
on this fixed block system. This is used on all lines
including the Victoria and Central Lines, which use also ATO (Automatic Train
Operation).
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Track circuits were first introduced to the UK in 1903 on
the Ealing and South Harrow Railway, now part of the Piccadilly Line branch to Rayners
Lane. The system was imported from the USA, where it had been tried on a number of
densely used routes. The track circuits in adjacent block sections were isolated
from each other by cutting the rails at the ends of the block and inserting insulated
joints. These became known as "insulated block joints".
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Fig. 3: Diagram of automatic signals
positioned to protect the entrance to block sections. Note the signals are
identified by the A prefix and are numbered like houses, odd numbers on one
side and even numbers on the other side.
Click on image for full size view
Signal Relays
Track Circuits
throughout, the single-rail type being used in
all cases-except on the Central London Railway, where both running rails are used for
traction current return, and, therefore, double-rail track circuits with impedance bonds
are used. The original track circuits for all lines (except the Central London Railway)
were direct current, each having two polarised track relays. The two relays were polarised
so that current could only pass one way in each. This prevented false energisation
of relays due to stray currents.
Alternating current track circuits were first
adopted on the Central London Railway, using single element vane relays. With
various new works and developments, different types of A.C. track circuits and relays have
been used, until to-day the standard track relay is of the two-element vane type, the
track circuit being of the direct, condenser fed type.
All signals are power-worked, and cabins are
only provided at places where there are junctions or points; most of the locking frames
are for power operation and are fitted with miniature levers. The signals worked from the
frames are of the semi- automatic type, being replaced to "danger" when a train
passes them, independently of the position of the lever. In most cases they are termed
"disengaged", that is, after being put to danger they will not return to the
clear position until the lever controlling them has been replaced to the normal posltton
in the power frame, and again reversed. This is accomplished by using what is known as a
"stick" relay.
The back-locking circuits for the electric
lock on the signal lever are also taken through this relay, and perform the following
functions : (1) If a train has approached to within a certain distance of the signal, and
the signal has been "pulled off" the lever cannot be put back to normal until
the train has passed beyond the clearing point of the junction, thus preventing a
signalman putting back his lever and resetting the road. (2) If, however, the lever is in
the reversed position, but the signal is at danger, due to the stick relay not having been
picked up, and a train approaches the signal, it is possible for the lever to be put to
normal, and pulled off again, owing to the back-lock circuit through the bottom contact on
the stick relay.
King Levers
To save( emPloyment of signalmen in the event
of the ten- Pdrary closing of the statiofi or siding controlled from a panicular cabin,
many of the signal-boxes have been Drovidcd with "king" levers. When pulled
over, the king lever locks the signal levers for the straight through roads in the reverse
position and thus enables the controlled section to be worked automatically.
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