Abstract: An interlocking system for a railway comprises a plurality of processors (A, B and C), the system having an input (1) for receiving input information and an output (2) for providing control information. Each of the processors is adapted to test itself to check that it is operating correctly and each of the processors is also adapted to test another of the processors to check that the other processor is operating correctly, each of the processors also being so tested by another of the processors. The system is shut down or put into a more restricted mode of operation if a fault in its operation is detected, either as a result of a processor's self-testing routine or as a result of one of the processors detecting that another processor is not operating correctly. This achieves the integrity of a "dual-channel" system with only a single "channel" of hardware.

Abstract: An AND function gating circuit for A.C. signals comprising several magnetic circuits around a common limb, each circuit having a separate input winding and a single output winding on the common limb. The magnetic flux level in the common limb is greatly increased when all input windings are simultaneously energized by A.C. signals of the same frequency and in phase relative to the output achieved under other input signal conditions.