Point Machines on London Underground
From the early days of electric traction and
automatic signalling on the London Underground, point machines were developed for
pneumatic operation. They have remained the principal type across the system ever
since. The main reason for their survival is that the air supply is already used for
trainstops and is therefore available and the speed of operation is twice that for
electric points. The following three photos show a standard LU points mechanism for
a set of points near Camden Town (Northern Line).
Fig. 1: Standard LU Four Foot Point Machine
This is the standard LU "four foot"
air operated points mechanism looking from the air cylinder end. The wooden plank
normally covering the mechanism is removed here to show the equipment. There is a
wooden ramp at each end which is provided to reduce the risk of damage from or to
equipment which might be hanging from trains. Photo by Tube Troll.
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Fig. 2: Four Foot Point Machine at the Ground
Lock End
This is the standard LU "four foot" air operated
points mechanism looking from the ground lock end. The wooden plank normally
covering the mechanism is removed. Photo by Tube Troll.
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Fig. 3: Four Foot Point Machine with Detector
Box Open
Points mechanism showing the mechanical and electrical
locking systems used to secure points in the correct position and verify the locking. Photo
by Tube Troll.
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