Interlocking
Encyclopedia
Railway signalling
Railway signalling is a system used to control railway traffic safely, essentially to prevent trains from colliding. Being guided by fixed rails, trains are uniquely susceptible to collision; furthermore, trains cannot stop quickly, and frequently operate at speeds that do not enable them to stop...
, an interlocking is an arrangement of signal apparatus that prevents conflicting movements through an arrangement of tracks such as junctions or crossings. The signalling 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".
Configuration and use
A minimal interlocking consists of signalsRailway signal
A signal is a mechanical or electrical device erected beside a railway line to pass information relating to the state of the line ahead to train/engine drivers. The driver interprets the signal's indication and acts accordingly...
, but usually includes additional appliances like switches
Railroad switch
A railroad switch, turnout or [set of] points is a mechanical installation enabling railway trains to be guided from one track to another at a railway junction....
(points in UK parlance), derails
Derail (railroad)
A derail or derailer is a device used to prevent fouling of a track by unauthorized movements of trains or unattended rolling stock. It works by derailing the equipment as it rolls over or through the derail....
, 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- the signal to proceed is withdrawn and sufficient time has passed to ensure that a train approaching that signal has had opportunity to come to a stop before passing the signal.
History
Railway interlocking is of BritishUnited Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
origin. Numerous patents were granted in Britain for manually operated interlocking devices in 1856, when John Saxby received the first patent for interlocking switches and signals, and 1867, when 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
London and North Western Railway
The London and North Western Railway was a British railway company between 1846 and 1922. It was created by the merger of three companies – the Grand Junction Railway, the London and Birmingham Railway and the Manchester and Birmingham Railway...
alone.
The first experiment with mechanical interlocking in the United States took place in 1874 by Messrs. Toucey and Buchanan at Spuyten Duyvil Junction in New York
New York City
New York is the most populous city in the United States and the center of the New York Metropolitan Area, one of the most populous metropolitan areas in the world. New York exerts a significant impact upon global commerce, finance, media, art, fashion, research, technology, education, and...
on the New York Central and Hudson River Railroad. The first important installations were the switches and signals of the Manhattan Elevated Railroad Company
New York City Transit Authority
The New York City Transit Authority is a public authority in the U.S. state of New York that operates public transportation in New York City...
and the New York Elevated Railroad Company in 1877-78.
An experimental hydro-pneumatic interlocking was installed at the Bound Brook, New Jersey
Bound Brook (New Jersey)
Bound Brook is a tributary of the Raritan River in Middlesex County, New Jersey in the United States.Its name comes from a boundary in an Indian deed. The stream is referred to as Sacunk, a Native American name meaning "slow sluggish stream", on early maps of the area.It rises in Edison and flows...
junction of the Philadelphia and Reading Railroad
Reading Company
The Reading Company , usually called the Reading Railroad, officially the Philadelphia and Reading Rail Road and then the Philadelphia and Reading Railway until 1924, operated in southeast Pennsylvania and neighboring states...
and the Lehigh Valley Railroad
Lehigh Valley Railroad
The Lehigh Valley Railroad was one of a number of railroads built in the northeastern United States primarily to haul anthracite coal.It was authorized April 21, 1846 in the U.S. state of Pennsylvania and incorporated September 20, 1847 as the Delaware, Lehigh, Schuylkill and Susquehanna 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
Union Switch & Signal
Union Switch and Signal was a supplier of railway signaling equipment, systems and services in Pittsburgh, Pennsylvania. As of January 1, 2009, US&S is known as Ansaldo STS USA.-History:...
(now a unit of Ansaldo STS
Ansaldo STS
Ansaldo STS S.p.A. is a multinational technology company which produces signalling and automation systems for use by rail and rapid transit operators. The firm also acts as lead contractor and turnkey provider on new rail developments...
, headquartered in Genoa, Italy), was first installed on the Chicago and Northern Pacific Railroad at its drawbridge across the Chicago River
Chicago River
The Chicago River is a system of rivers and canals with a combined length of that runs through the city of the same name, including its center . Though not especially long, the river is notable for being the reason why Chicago became an important location, as the link between the Great Lakes and...
. By 1900, 54 electro-pneumatic interlocking plants, controlling a total of 1,864 interlocking levers, were in use on 13 North American railroads and this type system would remain one of two viable competing systems into the future, although it did have the disadvantage of needing extra single-use equipment and requiring high maintenance.
“All-electric interlocking" became viable in May 1900, with the start of what became the General Railway Signal Company
General Railway Signal
General Railway Signal Company was a supplier of railway signaling equipment, systems and services in the Rochester, New York area. The company was established in 1904 and became part of Alstom Transport in 1998...
(now a unit of Alstom
Alstom
Alstom is a large multinational conglomerate which holds interests in the power generation and transport markets. According to the company website, in the years 2010-2011 Alstom had annual sales of over €20.9 billion, and employed more than 85,000 people in 70 countries. Alstom's headquarters are...
, headquartered in Levallois-Perret
Levallois-Perret
Levallois-Perret is a commune in the northwestern suburbs of Paris, France. It is located from the centre of Paris.-Name:The name Levallois-Perret comes from two housing developments, Champerret and Village Levallois , and which resulted in the incorporation of the...
, near Paris). The first installation of an all-electric interlocking plant was at Eau Claire, Wisconsin
Eau Claire, Wisconsin
Eau Claire is a city located in the west-central part of the U.S. state of Wisconsin. The population was 65,883 as of the 2010 census, making it the largest municipality in the northwestern portion of the state, and the 9th largest in the state overall. It is the county seat of Eau Claire County,...
on the Chicago, St. Paul, Minneapolis and Omaha Railway
Chicago, St. Paul, Minneapolis and Omaha Railway
The Chicago, St. Paul, Minneapolis and Omaha Railway or Omaha Road was a railroad in the U.S. states of Nebraska, Iowa, Minnesota, Wisconsin and South Dakota. It was incorporated in 1880 as a consolidation of the Chicago, St. Paul and Minneapolis Railway and the North Wisconsin Railway,. The...
in 1901. By 1913, this type system had been installed on 83 railroads in 35 US States and Canadian Provinces, in 440 interlocking plants using 21,370 levers.
Interlocking types
Interlockings can be categorized as mechanical, electrical (relayRelay
A relay is an electrically operated switch. Many relays use an electromagnet to operate a switching mechanism mechanically, but other operating principles are also used. Relays are used where it is necessary to control a circuit by a low-power signal , or where several circuits must be controlled...
-based), or electronic/computer-based.
Mechanical interlocking
Lever
In physics, a lever is a rigid object that is used with an appropriate fulcrum or pivot point to either multiply the mechanical force that can be applied to another object or resistance force , or multiply the distance and speed at which the opposite end of the rigid object travels.This leverage...
s that operate switches
Railroad switch
A railroad switch, turnout or [set of] points is a mechanical installation enabling railway trains to be guided from one track to another at a railway junction....
, derails
Derail (railroad)
A derail or derailer is a device used to prevent fouling of a track by unauthorized movements of trains or unattended rolling stock. It works by derailing the equipment as it rolls over or through the derail....
, 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
Bricklayers' Arms
Bricklayers' Arms is a busy road intersection between A2 and the London Inner Ring Road in south London, England. It is the junction of Tower Bridge Road, Old Kent Road, New Kent Road and Great Dover Street; Old Kent Road and New Kent Road east-bound are connected by a flyover.The area is named...
, 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-pneumaticallyPneumatics
Pneumatics is a branch of technology, which deals with the study and application of use of pressurized gas to effect mechanical motion.Pneumatic systems are extensively used in industry, where factories are commonly plumbed with compressed air or compressed inert gases...
. 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 General Railway Signal
General Railway Signal
General Railway Signal Company was a supplier of railway signaling equipment, systems and services in the Rochester, New York area. The company was established in 1904 and became part of Alstom Transport in 1998...
(GRS) power interlocking machine.
Relay interlocking
Relay logic
Relay logic is a method of controlling industrial electronic circuits by using relays and contacts.-Ladder logic:The schematic diagrams for relay logic circuits are often called line diagrams, because the inputs and outputs are essentially drawn in a series of lines...
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
Fail-safe
A fail-safe or fail-secure device is one that, in the event of failure, responds in a way that will cause no harm, or at least a minimum of harm, to other devices or danger to personnel....
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
Lincoln, Nebraska
The City of Lincoln is the capital and the second-most populous city of the US state of Nebraska. Lincoln is also the county seat of Lancaster County and the home of the University of Nebraska. Lincoln's 2010 Census population was 258,379....
on the Chicago, Burlington and Quincy Railroad
Chicago, Burlington and Quincy Railroad
The Chicago, Burlington and Quincy Railroad was a railroad that operated in the Midwestern United States. Commonly referred to as the Burlington or as the Q, the Burlington Route served a large area, including extensive trackage in the states of Colorado, Illinois, Iowa, Kentucky, Missouri,...
.
Entrance-Exit Interlocking (NX) was the original brand name of the first generation relay-based centralized traffic control
Centralized traffic control
Centralized traffic control is a form of railway signalling that originated in North America and centralizes train routing decisions that were previously carried out by local signal operators or the train crews themselves. The system consists of a centralized train dispatcher's office that...
(CTC) interlocking system introduced in 1936 by GRS (represented in Europe by Metropolitan-Vickers
Metropolitan-Vickers
Metropolitan-Vickers, Metrovick, or Metrovicks, was a British heavy electrical engineering company of the early-to-mid 20th century formerly known as British Westinghouse. Highly diversified, they were particularly well known for their industrial electrical equipment such as generators, steam...
). 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, and 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
Brunswick railway station
Brunswick railway station is a railway station in Dingle, Liverpool, England, on the Northern Line of the Merseyrail rapid-transit urban network. It also serves the nearby district of Dingle and is situated on a short section of track between two tunnels, between the now in-filled Toxteth and...
on the Cheshire Lines
Cheshire Lines Committee
The Cheshire Lines Committee was the second largest joint railway in Great Britain, with 143 route miles. Despite its name, approximately 55% of its system was in Lancashire. In its publicity material it was often styled as the Cheshire Lines Railway...
, UK. The first US installation was on the New York Central Railroad
New York Central Railroad
The New York Central Railroad , known simply as the New York Central in its publicity, was a railroad operating in the Northeastern United States...
(NYCRR) at Girard Junction, Ohio in 1937. Another NYCRR installation was on the main line between Utica, New York
Utica, New York
Utica is a city in and the county seat of Oneida County, New York, United States. The population was 62,235 at the 2010 census, an increase of 2.6% from the 2000 census....
and Rochester, New York
Rochester, New York
Rochester is a city in Monroe County, New York, south of Lake Ontario in the United States. Known as The World's Image Centre, it was also once known as The Flour City, and more recently as The Flower City...
, and this was quickly followed up by three installations on the New York City Transit System
New York City Transit Authority
The New York City Transit Authority is a public authority in the U.S. state of New York that operates public transportation in New York City...
in 1948.
Other NX style systems were implemented by other railroad signal providers. For example Union Route (UR) was the brand name of their vacuum tube
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
-based NX overlay system supplied by Union Switch & Signal Co.
Union Switch & Signal
Union Switch and Signal was a supplier of railway signaling equipment, systems and services in Pittsburgh, Pennsylvania. As of January 1, 2009, US&S is known as Ansaldo STS USA.-History:...
(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
Telephone exchange
In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...
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 stateSolid state (electronics)
Solid-state electronics are those circuits or devices built entirely from solid materials and in which the electrons, or other charge carriers, are confined entirely within the solid material...
, 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. In many implementations this vital logic is stored as firmware
Firmware
In electronic systems and computing, firmware is a term often used to denote the fixed, usually rather small, programs and/or data structures that internally control various electronic devices...
or in ROM
Rom
ROM, Rom, or rom is an abbreviation and name that may refer to:-In computers and mathematics:* Read-only memory, a type of storage media which is used in computers and other electronic devices....
that cannot be easily altered to both resist unsafe modification and meet regulatory safety testing requirements.
At this time there were also changes in the systems that controlled interlockings. Whereas before technologies such as NX and Automatic Route Setting required racks and racks of relays and other devices, solid state software based systems could handle such functions with less cost and physical footprint. Initially processor driven Unit Lever and NX panels could be set up to command field equipment of either electronic or relay type, however as display technology improved, these hard wired physical devices could be updated with visual display units, which allowed changes in field equipment be represented to the signaller without any hardware modifications.
Solid State Interlocking
Solid State Interlocking
Solid State Interlocking is the brand name of the first generation processor-based interlocking developed in the 1980s by British Rail's Research Division, GEC-General Signal and Westinghouse Signals Ltd in the UK.- Interlocking hardware :...
(SSI) is the brand name of the first generation microprocessor
Microprocessor
A microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...
-based interlocking developed in the 1980s by British Rail
British Rail
British Railways , which from 1965 traded as British Rail, was the operator of most of the rail transport in Great Britain between 1948 and 1997. It was formed from the nationalisation of the "Big Four" British railway companies and lasted until the gradual privatisation of British Rail, in stages...
, GEC-General Signal and Westinghouse Signals Ltd
Westinghouse Rail Systems
Westinghouse Rail Systems Ltd is a British supplier of railway signalling and control equipment to the rail industry worldwide. Its head office is in Chippenham, Wiltshire, where it manufactures a variety of mechanical and electrical/electronic railway signalling equipment...
in the UK. Second generation processor-based interlockings are known by the term "Computer Based Interlocking" (CBI), of which MicroLok (trademark of Union Switch & Signal, now Ansaldo STS), Westlock
Westlock Interlocking
WESTLOCK Interlocking is a Solid State Interlocking product by Westinghouse Rail Systems.Westlock builds on many of the features that made SSI popular in the United Kingdom. This includes re-use of SSI's programming language and its external hardware....
and Westrace (trademarks of Invensys Rail) and Smartlock (trademark of Alstom
Alstom
Alstom is a large multinational conglomerate which holds interests in the power generation and transport markets. According to the company website, in the years 2010-2011 Alstom had annual sales of over €20.9 billion, and employed more than 85,000 people in 70 countries. Alstom's headquarters are...
) 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 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."
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.