Why is double cutting employed in railway signaling?

This is employed to protects against earth faults.As railway signalling equipment is designed to fail safe it is most important that stray currents due to earth faults do not cause false operation of equipment.If an earth fault occurs on both positive legs of a circuit then the repeating relays will be false fed by the track relays being energised.To stop this dangerous situation from occurring circuitry is designed with contacts in both positive and negative legs. (DOUBLE CUTTING).Now if earth faults exist the circuits operate safely because the negative supply has been cut by the additional Track Relay contact.