A computer based railway interlocking control system, preferably of the parallel redundant type (Fig. 2) in order to provide enhanced reliability as compared to a single computer system. The railway traffic control devices, e.g. points and signals, are arranged to be controlled by signals in one of two possible steady states, the interlocking control means (30, 31, 32, Fig. 2) is arranged to produce control signals in the steady state for de-energised or "safe" device settings and in an alternating state for energised or potentially "unsafe" settings. The alternating control signals (33, 34, 35 or 38) must be converted to the corresponding steady state to operate the devices thus, the converting or restoring circuit discriminates against dormant wrong side failures, i.e. "unsafe" signals to increase the inherent safety of the system