If a facing point is immediately beyond a signal (typically within 50yds/45.7m), it is possible that a train passing the signal at danger could become derailed on the points, causing damage to both train and track. Although points have dead locking (where occupancy of the track section over the point locks the point in its current position), there may not be enough time for the interlocking to apply the dead locking.
Thus, the points typically have conditions of (track X clear or track X occupied for time). Track X is the berth track of the signal, ie the track section immediately preceding the track section with the point. Track clear means there is no train so it is safe to move the points. Track occupied for time, with the time dependent on the track section length and speed limit, times a train to a stand, thus meaning it is now (probably) safe to move the points again.
The time out is usually the same as that for releasing any overlap over the points. Therefore, with a train approaching, the overlap is free to swing until the train occupies the last track section before the point. The points now become locked in their current position until the time out expires, whereupon the overlap releases and the point becomes free to move again.
Last edited by GeoffM on 15/09/2016 at 03:01