Overlength indicators are in place in some boxes to control access by a second train to an occupied (mainly terminus) platform. The actual operation between boxes employs the same general principle but the actual operation may differ. It will not prevent occupation by the first train even if it is in itself overlength.
Essentially these indicators use measuring equipment to approximate the length of the occupying train and of the approaching train. If a train is signalled to an occupied platform, the signalling will detect if the remaining available length is enough to fit it.
If not then a panel indication is given and the subsidiary signal will not clear.
This is not to be relied upon and it must be remembered that the system only approximates train lengths often by detecting occupation of Track Circuits of a known length - but if a driver stops short (say) a false conclusion may be reached e.g. if a 250ft TC is occupied by one axle it is assumed the entire TC is fully occupied by a 250ft train.
Last edited by GeoffM on 15/09/2016 at 03:01