Wales from the 1900s, shows an ever increasing proportion of double switching compared to single switching. Double switching does of course cost more wires, more relay contacts, and testing. On the other hand double switching is inherently less prone to wrong side failures; it helps overcome short-circuit faults that are hard to test for. Partial double switching might double switch the lever controls, and the track circuits between one signal and the next, while single switching the track circuits in the less critical overlap beyond the next signal. Double switching is facilitated by more modern relays that have more contacts in less space: Pre-1950 Shelf Type Relay - 12 contacts (front (make) and back (break)) - full size Post-1950 Q-type plug in relay - 16 contacts (front (make) and back (break)) - about half size See also Redundancy (engineering) Double insulation Single-wire earth return References Couplers Railway signalling Safety Fault tolerance