Clive D. W. Feather wrote:

You mean "or apparent reason".

Indeed.


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.


True. District Dave once described a track circuit failure involving
a broken wire; he was involved in its repair.


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.

I don't know much about the trackside arrangements for track circuits,
so I accept what you've said.

Is it possible for a wet track circuit to short out in such a way that
a green aspect could be obtained? You've stated that track circuits
require a pair of false feeds to turn green; in the case of East Putney
- Wimbledon, what would need to get wet to cause such a wrong-side
failure?