Editing Double switching (section)

== Signalling in NSW ==

A study of railway electrical signalling in New South 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 (railway signalling)|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#Railway signalling|relay]] - 16 contacts (front (make) and back (break)) - about half size