InterlockingIn railway signaling, an interlocking is an arrangement of signal apparatus that prevents conflicting movements through an arrangement of tracks such as junctions or crossings。 The signaling appliances and tracks are sometimes collectively referred to as an interlocking plant。 An interlocking is designed so that it is impossible to give clear signals to trains unless the route to be used is proved to be safe。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 。Interlocking typesInterlockings can be categorized as mechanical, electrical (relay-based), or electronic/computer-based。Mechanical interlockingIn 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)。Electro-mechanical interlockingPower 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, an indication must be received from the field element that it has actually moved into the position requested。 Relay interlockingInterlockings effected purely electrically (sometimes referred to as all-electric) consist of complex circuitry made up of relays 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。 Furthermore, such an interlocking may 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。“Entrance-Exit Interlocking (NX)” was the original brand name of the first generation tube relay based Centralized Traffic Control (CTC) interlocking system developed in the 1940s by GRS 。Its first installation was on the New York Central Railroad main line between Utica, New York and Rochester, New York, and this was quickly followed up by three installations on the New York City Transit System in 1948：On their Fulton Street-8th Avenue Line： Between Shepherd Avenue station and the end of track 1,569 feet south of Euclid Avenue station。 Between Euclid Avenue station and the end of track in Pitkin Yard。 This yard had 44 tracks at the time, and three ladders。 On their Queens Boulevard Line between the 169th Street station and the end of track 1,584 feet north of 179th Street station on both levels。 The success of these installations resulted in the New York Central RR adding additional installations as money permitted, while NYCTS cancelled all further orders for relay interlocking systems, and has installed only electronic systems since then。Relay interlocking systems were often used in large and busy stations that have to handle high volumes of train movements。 Since the 1980s, new interlockings have tended to be of the electronic variety, which effect electronically what was previously effected with relays。Electronic interlockingModern 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。 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。Regardless of the technology used, interlockings are designed to ensure that