Interlockings can be categorized as mechanical, electrical (electro-mechanical or
relay-based), or electronic/
computer-based.
Mechanical interlocking 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. The bars are constructed so that if the function controlled by a given lever conflicts with that controlled by another lever, mechanical interference is set up in the
cross locking between the two bars, in turn preventing the conflicting lever movement from being made. In purely mechanical plants, the levers operate the field devices, such as signals, directly via a mechanical rodding or wire connection. The levers are about shoulder height since they must supply a mechanical advantage for the operator. Cross locking of levers was effected such that the extra leverage could not defeat the locking (preliminary latch lock). The first mechanical interlocking was installed in 1843 at
Bricklayers Arms Junction, England.
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 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 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 ==