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In 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".


Railroad interlocking is of Britishmarker origin. Numerous patents were granted in Britain for manually operated interlocking devices from 1856, when John Saxby received the first patent for interlocking switches and signals to 1867, when Saxby (of Saxby & Farmer) was awarded a patent for what is known today in North America as “preliminary latch locking”. Mechanical interlocking improved when 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 that time, not one interlocking lever was in use in the United States.

The first experiment with interlocking in the United States took place the following year by Messrs. Toucey and Buchanan at Spuyten Duyvil Junction in New Yorkmarker on the New York Central and Hudson River Railroad. The first important installation were the switches and signals of the Manhattan Elevated Railroad Company and the New York Elevated Railroad Company in 1877-78.

The next step took place on the American side of the Atlantic Ocean. An experimental hydro-pneumatic interlocking was installed at the Bound Brook, New Jersey junction of the Philadelphia and Reading Railroad and the Lehigh Valley Railroad in 1884. By 1891, there were 18 such hydro-pneumatic plants, on six railroads, operating a total of 482 levers. The installations worked, but there were serious defects in the design, and little saving of labor was achieved. The inventors of the hydro-pneumatic system moved forward to an electro-pneumatic system in 1891 and this system, best identified with the Union Switch & Signal Company, was first installed on the Chicago and Northern Pacific Railroad at their drawbridge across the Chicago Rivermarker.

By 1 June 1900, 54 electro-pneumatic interlocking plants, controlling a total of 1,864 interlocking levers, were in use on 13 railroads and the system would remain one of two viable competing systems into the future, although it does have the disadvantage of requiring extra single-use equipment and high maintenance.

Work on putting together the components of what today is known as "all-electric interlocking" became viable in May 1900, with the start of the assembly of what became the General Railway Signal Company marker. General Railway Signal (GRS) was founded in 1904 with the merger of three companies (Pneumatic Signal Company of Rochester, New York, Taylor Signal Co. of Buffalo, New York and Standard Railroad Signal Company of Arlington, New Jersey). The first installation of an all-electric interlocking plant was at Eau Claire, Wisconsinmarker on the Chicago, St. Paul, Minneapolis and Omaha Railroad in 1901. By 1913, the system had been installed on 83 railroads in 35 US States and Canadian Provinces on 440 interlocking plants, using 21,370 levers.

There have been railway track in the province of Saskatchewanmarker. Rail companies were intersected 58 times at level crossings. Some of these were railway lines crossing municipal street car rail lines. There were 36 Saskatchewan crossings which were controlled by mechanical interlocking machines between the early 1900s and 1990.

In the 16 years from the installation of the first commercial pneumatic machine, during which time no competitive power interlocking machine was on the market, the average annual sales were 4.5 machines and 147 levers. In the 11 years following the installation of the first commercial all-electric interlocking machine, the average annual sales were 40 machines and 1,943 levers.

GRS soon became the majority manufacturer, with US&S taking the number two position, a situation that remained for the remainder of the life of the two companies. Both companies and their successors compete for all-electric and pneumatic systems, and there are installations where compromise systems consisting of components from both companies (either all-electric or mixed all-electric and electro-pneumatic) exist. This is especially true on MTA New York City Transit.

Configuration and use

A model board and lever machine
A minimal interlocking consists of signals, but usually includes additional appliances like switches (points in UK parlance), derails, 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.
  • 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 signal has had opportunity to come to a stop before passing the signal.

Interlocking types

Interlockings can be categorized as mechanical, electrical (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).

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, an indication 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

Part of a relay interlocking
Interlockings 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 GRSmarker (represented in Europe by Metropolitan Vickers). Its first installation was on the New York Central Railroad main line between Utica, New Yorkmarker and Rochester, New Yorkmarker, 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:
  1. Between Shepherd Avenue station and the end of track 1,569 feet south of Euclid Avenue station.
  2. 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.

Union Route (UR) was the brand name of Union Switch and Signal (US&S)’s competing system of Tube Based Electronic Interlocking, which was introduced in 1951.

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 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. 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 no operation can be performed unless all prerequisites have been satisfied.

"Solid State Interlocking" (SSI) is the brand name of the first generation processor-based interlocking developed in the 1980s by British Rail, GEC-General Signal and Westinghouse Signals Ltd in the UK. Second generation processor-based interlockings are known by the term "Computer Based Interlocking" (CBI), of which MicroLok, Westlock and Smartlock - a Trademark from ALSTOM Transport - are examples.

Defined forms of locking

  • Electric locking:
:“The combination of one or more electric locks and controlling circuits by means of which levers in an interlocking machine, or switches or other devices operated in connection with signaling 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”.

  • Route locking:
:“Electric locking taking effect when a train passes a signal and adapted to prevent manipulation of levers that would endanger the train while it is within the limits of the route entered”.

  • Sectional route locking:
:“Route locking so arranged that a train, in clearing each section of the route, releases the locking affecting that section”.

  • Approach locking:
:“Electric locking effective while a train is approaching a signal that has been set for it to proceed and adapted to prevent manipulation of levers or devices that would endanger that train”.

  • Stick locking:
:“Electric locking taking effect upon the setting of a signal for a train to proceed, released by a passing train, and adapted to prevent manipulation of levers that would endanger an approaching train”.

  • Indication locking:
:“Electric locking adapted to prevent any manipulation of levers that would bring about an unsafe condition in case a signal, switch, or other operated device fails to make a movement corresponding with that of the operating lever; or adapted directly to prevent the operation of one device in case another device, to be operated first, fails to make the required movement”.

  • Check locking or traffic locking:
:“Electric locking that enforces cooperation between the Operators at two adjacent plants in such a manner that prevents opposing signals governing the same track from being set to proceed at the same time. In addition, after a signal has been cleared and accepted by a train, check locking prevents an opposing signal at the adjacent interlocking plant from being cleared until the train has passed through that plant”.

Complete and incomplete interlockings (U.S. terminology)

Interlockings allow trains to cross from one track to another using a "turnout" and a series of switches. Railroad terminology defines the following types of Interlockings as either "complete" or "incomplete" depending on the movements available. Although timetables generally do not identify an interlocking as one or the other, and rule books do not define the terms, the below is generally agreed upon by system crews and rules officials.

Complete interlockings allow continuous movements from any track on one side of the interlocking to any track on the opposite side without the use of a reverse move within the limits of the interlocking. This is true even if there are differing numbers of tracks on opposing sides, or if the interlocking has multiple sides.

Incomplete interlockings do not allow such movements as described above. Movements in an incomplete interlocking may be limited and may even require reverse movements to achieve the desired route.

See also


  1. Josserand, Peter; Rights Of Trains; Simmons-Boardman Publishing Corp; New York, New York; 1951; Definitions, P. 5.
  2. General Railway Signal Company; Electric Interlocking Handbook; 1913; General Railway Signal Company; Rochester, New York; Pps. 5-12.
  3. The first manufacturer of signal equipment, the predecessor of Westinghouse Brake and Signal Company Ltd, and today’s Westinghouse Rail Systems, Ltd.
  4. A system whereby compressed water and air are used to transmit action from one end of a long tube to the other end. It can be effective, but it still qualifies as a mechanical system since the pressure is pre-loaded, and requires human action of the same sort that a pure mechanical system requires.
  5. NYCTA General Signal Arrangement Drawing MW7A-411-SL
  6. NYCTA General Signal Arrangement Drawing MW7A-10-SL
  7. NYCTA General Signal Arrangement Drawing MW7D-1726-SL and MW-7D-1764-SK
  8. *Patent; Mccann, Ronald A.; 1951; Entrance-exit route interlocking control apparatus; United States; UNION SWITCH & SIGNAL CO; #2567887; [1]
  9. Railway Group Guidance Note GK/GN0802
  10. Defined by the Railway Signal Association, which today is the Railway Signal Committee of the American Association of Railroads

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