thumb|The tower and tracks at Deval interlocking, [[Des Plaines, Illinois, in 1993]]
In railway signalling, an interlocking is an arrangement of signal apparatus that prevents conflicting movements through an arrangement of tracks such as junctions or crossings. In North America, a set of signalling appliances and tracks interlocked together are sometimes collectively referred to as an interlocking plant or just as an 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. For example, interlocking would prevent a signal from being changed to indicate a diverging route, unless the corresponding points/switches had been changed first. In North America, the official railroad definition of interlocking is: "An arrangement of signals and signal appliances so interconnected that their movements must succeed each other in proper sequence".
Configuration and use
thumb|A model board and lever machine
A minimal interlocking consists of signals, but usually includes additional appliances such as points and Facing Point locks (US: switches) and derails, and may include crossings at grade and movable bridges. Some of the fundamental principles of interlocking include:
- Signals may not be operated to permit conflicting train movements to take place at the same time on set route.
- Switches and other appliances in the route must be properly 'set' (in position) before a signal may allow train movements to enter that route.
- Once a route is set and a train is given a signal to proceed over that route, all switches and other movable appliances in the route are locked in position until either
- the train passes out of the portion of the route affected, or
- the signal to proceed is withdrawn and sufficient time has passed to ensure that a train approaching that route has had opportunity to come to a stop before passing the signal.
History
Railway interlocking is of British origin, where numerous patents were granted. In June 1856, John Saxby received the first patent for interlocking switches and signals. In 1868, Saxby (of Saxby & Farmer) was awarded a patent for what is known today in North America as “preliminary latch locking”. Preliminary latch locking became so successful that by 1873, 13,000 mechanical locking levers were employed on the London and North Western Railway alone. At the time, Toucey was General Superintendent and Buchanan was Superintendent of Machinery on the NYC&HRR. Toucey and Buchanan formed the Toucey and Buchanan Interlocking Switch and Signal Company in Harrisburg, Pennsylvania in 1878. The first important installations of their mechanism were on the switches and signals of the Manhattan Elevated Railroad Company and the New York Elevated Railroad Company in 1877–78. interlocking was installed at the Bound Brook, New Jersey junction of the Philadelphia and Reading Railroad and the Lehigh Valley Railroad in 1884. Another interlocking of this type was installed in Westend near Berlin in 1896. 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).
Electro-mechanical interlocking
Power interlockings may also use mechanical locking to ensure the proper sequencing of levers, but the levers are considerably smaller as they themselves do not directly control the field devices. If the lever is free to move based on the locking bed, contacts on the levers actuate the switches and signals which are operated electrically or electro-pneumatically. Before a control lever may be moved into a position which would release other levers, a signal must be received from the field element that it has actually moved into the position requested. The locking bed shown is for a GRS power interlocking machine.
Relay interlocking
thumb|Part of a relay interlocking using miniature plug-in relays
Interlockings effected purely electrically (sometimes referred to as all-electric) consist of complex circuitry made up of relays in an arrangement of relay logic that ascertain the state or position of each signal appliance. As appliances are operated, their change of position opens some circuits that lock out other appliances that would conflict with the new position. Similarly, other circuits are closed when the appliances they control become safe to operate. Equipment used for railroad signalling tends to be expensive because of its specialized nature and fail-safe design.
Interlockings operated solely by electrical circuitry may be operated locally or remotely, with the large mechanical levers of previous systems being replaced by buttons, switches or toggles on a panel or video interface. Such an interlocking may also be designed to operate without a human operator. These arrangements are termed automatic interlockings, and the approach of a train sets its own route automatically, provided no conflicting movements are in progress.
GRS manufactured the first all-relay interlocking system in 1929. It was installed in Lincoln, Nebraska on the Chicago, Burlington and Quincy Railroad. (represented in Europe by Metropolitan-Vickers). The advent of all electric interlocking technology allowed for more automated route setting procedures as opposed to having an operator line each part of the route manually. The NX system allowed an operator looking at the diagram of a complicated junction to simply push a button on the known entrance track and another button on the desired exit track. The logic circuitry handled all the necessary actions of commanding the underlying relay interlocking to set signals and throw switches in the proper sequence, as required to provide valid route through the interlocking plant. The first NX installation was in 1937 at Brunswick on the Cheshire Lines, UK. The first US installation was on the New York Central Railroad (NYCRR) at Girard Junction, Ohio in 1937.
Other NX style systems were implemented by other railroad signal providers. For example, Union Route (UR) was the brand name of their Entrance-Exit system supplied by Union Switch & Signal Co. (US&S), and introduced in 1951. 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. Such advanced schemes would also include train describer and train tracking technologies. Away from complex terminals unit lever control systems remained popular until the 1980s when solid state interlocking and control systems began to replace the older relay plants of all types.
Electronic interlocking
thumb|Computer-based controls for a modern electronic interlocking
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. of which VPI (trademark of General Railway Signal, now Alstom), MicroLok (trademark of Union Switch & Signal, now Hitachi Rail STS), Westlock and Westrace (trademarks of Invensys Rail, now Siemens), Smartlock (trademark of Alstom), and EBI Lock (trademark of Bombardier) are examples.
Defined forms of locking
; Electric locking : "The combination of one or more electric locks or controlling circuits by means of which levers in an interlocking machine, or switches or other devices operated in connection with signalling and interlocking, are secured against operation under certain conditions."
; Section locking : "Electric locking effective while a train occupies a given section of a route and adapted to prevent manipulation of levers that would endanger the train while it is within that section."