de-energised

Alternatively, they clear when the line is unoccupied to the next signal and when an approaching train occupies a short, normally de-energised, track circuit on the approach side of any of the signals. These normally de-energised track circuits are labelled with a Greek letter to emphasise their difference from the normal track circuits, and are short enough to ensure an adequate restriction of speed being imposed on the train.

1952 September, Norman Crump, “Camden Town Signalbox, London Transport”, in Railway Magazine, page 585