London underground cables

[David H]

When the track is already live, why are there maybe twenty or more cables running alongside LU tracks? What does each one of them do besides power the actual train?

[qapmoc]

Some will be power cables, the tracks are not all connected together as one continuous circuit, but divided into ''sections'' so most of the cables are supply for the various sections and return power lines also, the others will be multicore signal cables and communications cables.

[Cathyhuns]

The LU track (or running rails) are not live. The electricity to power the trains (or traction power) is carried in the other two rails on the white ceramic supports, one outside the two running rails and one in the middle. The traction power is 650V dc and there are two rails so that you don't get current leakage to ground.

The other wires or cables carry power to the signalling system, lights and other systems that need ac power. There are also several communication systems such as radio, tunnel telephone and general LU private telephone. There are also the safety bsystems which let the signallers know where the trains are and cables to support the "safety brakes" the failure of which caused all the problems on the Northern Line before Christmas.

There are also rumours of MI5 and other communication systems using the Tube for their cables but that has never been proven.