Signalling and Trainstop Photos
Fig. 1: Automatic Signal with
Trainstop
Typical London Underground automatic signal
(on the left) with its trainstop in the raised position, placed at the right hand side of
the track. The trainstop is lowered when the signal shows a clear (green)
aspect. Compressed air is used to lower the trainstop against the spring pressure
used to raise it. This provides an element of fail safe, should the air supply be
lost. The air is compressed in the traction sub-stations and is distributed
alongside the track in an air main. This can be seen as the silver pipe in the photo
above supported on posts at the track side, together with the signalling and power
cables. Photo taken at Ruislip Manor (Metropolitan).
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Fig. 2: Block Joints and
Trainstop
This photo shows the insulated block joints in the running
rails at the end of a block section. Note the raised trainstop on the right hand
side of the track acting with a red signal (not seen here) to protect the entrance to the
next block.
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. This is where this photo was taken.
A problem with block joints is that the surface
of the rail spreads over time with the intensive use, forming "scaling" over the
block joint. This causes the two adjacent sections to become electrically connected
and results in the "track circuit failure" so often heard about.
The plate on the
track shown in the photo above is a stopping mark provided for train drivers indicating
this is the place where 8-car trains should stop.
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Fig. 3: Tripcock Tester
This photo shows a tripcock tester (Acton Town, Piccadilly
Line) with the names of parts. The tripcock tester is provided to check that the
tripcocks of passing trains are in operative condition and within the correct gauge to
strike a raised trainstop.
As the train passes over the tester, the tripcock arm should
pass through the two vertical gauge posts at the entrance to the tester. If it does
not, and the tripcock hits one of the posts, the train will be tripped and will come to a
stand.
If the tripcock arm passes through the gauge successfully,
the arm will depress a lightly sprung ramp. As the ramp depresses, a circuit is
completed to indicate the test was successful. The indication consists of a small
light (either white, blue or purple, depending on location) mounted near the starting
signal. The light is switched on automatically as the train approaches the tester
and the light will go out if the ramp is depressed correctly.
If the train passes through the ramp too fast, it is likely
to get tripped anyway. For this reason, tripcock testers are invariably located in
station platforms.
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