In article om,
TheOneKEA writes
"Bobbing" is when the signal rapidly changes from aspect to aspect
without warning or reason.

You mean "or apparent reason".

By far the commonest reason for bobbing is that one of the controlling
track circuits is being intermittently shorted out or failing open
circuit. For example, if one of the wires bonding adjacent rails is
loose, the circuit can break and re-make under the vibration of trains
on adjacent tracks.

Naturally this is a major problem because a signal could bob despite an
occupied track in front of the train; thus, if it goes to green and the
driver passes it in the correct way at speed, CRUNCH.

Bobbing *to* green is extremely unlikely, because it requires a false
feed at an appropriate point (and circuits are usually designed to
require two false feeds for a wrong-side failure). There's no obvious
situation, comparable to the one above, to cause this.

--
Clive D.W. Feather | Home:
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