Speed Control Photos
An example of how the driver sees speed
control signals operating.
Fig.1: The Approach to Acton Town.
This is the driver's view of the eastbound approach to Acton
Town (Piccadilly Line) when coming from Ealing Common. He is shown four red signals
indicating that the platform ahead is occupied. The first signal will be cleared if
the driver controls the train so that its speed is detected as being low enough to enable
a stop at the second signal.
The short section of "current rail" on the right
hand side is the instantaneous speed detector. This contains a series of
electro-magnetic coils of alternating polarity, mounted on a magnetic base plate, which
are activated as the trains current collector shoe passes over it. As the shoe
passes, an alternating current is generated with a frequency proportional to the speed of
the train. This is passed through a frequency filter and, if within the correct
range, a relay is activated to clear the signal. Photo by Tube Troll.
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Fig.2: The Signal Clearing Sequence
In this second photo, the driver has reduced the speed of the
train, the first, speed controlled signal has cleared and the train is now approaching the
next signal. This signal has also cleared to green because the train in the platform
has started to leave. The remaining signals will now clear in sequence. Photo
by Tube Troll.