Interlocking
An interlocking system is designed so that it is impossible to display a signal to proceed unless the route to be used is proven safe.
Interlocking is a safety measure designed to prevent signals and points/switches from being changed in an improper sequence.
[6] Compared to Saxby's design, Toucey and Buchanans' interlocking mechanism was more cumbersome and less sophisticated, and so was not implemented very widely.
[8] As technology advanced the railway signaling industry looked to incorporate these new technologies into interlockings to increase the speed of route setting, the number of appliances controlled from a single point and to expand the distance that those same appliances could be operated from the point of control.
The challenge facing the signal industry was achieving the same level of safety and reliability that was inherent to purely mechanical systems.
[11] In North America, the first installation of an interlocking plant using electric switch machines was at Eau Claire, Wisconsin on the Chicago, St. Paul, Minneapolis and Omaha Railway in 1901, by General Railway Signal Company (GRS, now a unit of Alstom, headquartered in Levallois-Perret, near Paris).
[7] By 1913, this type system had been installed on 83 railroads across 35 US states and Canadian provinces, in 440 interlocking plants using 21,370 levers.
In mechanical interlocking plants, a locking bed is constructed, consisting of steel bars forming a grid.
The levers that operate switches, derails, signals or other appliances are connected to the bars running in one direction.
[16] NX type systems and their costly pre-solid state control logic only tended to be installed in the busier or more complicated terminal areas where it could increase capacity and reduce staffing requirements.
In a move that was popular in Europe, the signalling for an entire area was condensed into a single large power signal box with a control panel in the operator's area and the equivalent of a telephone exchange in the floors below that combined the vital relay based interlocking logic and non-vital control logic in one place.
Modern interlockings (those installed since the late 1980s) are generally solid state, where the wired networks of relays are replaced by software logic running on special-purpose control hardware.
[3]: 84 The fact that the logic is implemented by software rather than hard-wired circuitry greatly facilitates the ability to make modifications when needed by reprogramming rather than rewiring.
In many implementations, this vital logic is stored as firmware or in ROM that cannot be easily altered to both resist unsafe modification and meet regulatory safety testing requirements.
