Is approach release fail safe?

[nagaraju_dvv]

In London Underground, they use Delta Rail Circuit for approach control. Delta Rail Circuit is a 10kHz open circuit superimposed on existing AC track circuit,whose length is small. The Delta Circuit is closed when shunted by wheels of train. The exact location of train on the approach to a signal can be verified by energisation of delta relay and the front contact of this delta relay is used to clear the signal. In this way this method used on LU is fail safe because failure of delta relay means signal will not clear. But coming to UK Main line signalling there appears to be different techniques used for approach control, each having its own merits and de-merits which I need to investigate further.

If the track circuit that controls the release of the aspect is failed occupied, remember that it is one of the tracks in the line of route of the signal in rear, so the signal in rear will not clear up to the approach released signal if this failure condition is present.

Of course you could have the TC operating normally and when the train has gone past the signal in rear, the TC MAY fail as the train comes along and release the signal early, but you will have had the yellow in the signal in rear.