Solid State Interlocking
Encyclopedia
Interlocking
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. The signalling appliances and tracks are sometimes collectively referred to as an interlocking plant...
developed in the 1980s by British Rail's Research Division
British Rail Research Division
The British Rail Research Division came into being in 1964 directly under the control of the British Railways Board, moving into purpose-built premises at the Railway Technical Centre in Derby. The intention was to improve railway reliability and efficiency, while reducing costs and improving revenue...
, GEC-General Signal
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...
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.
Interlocking hardware
SSI utilises a 2-out-of-3 redundancy architecture, whereby all safety-criticalLife-critical system
A life-critical system or safety-critical system is a system whose failure ormalfunction may result in:* death or serious injury to people, or* loss or severe damage to equipment or* environmental harm....
functions are performed in three separate processing lanes and the results voted upon. An SSI interlocking cubicle comprises three Interlocking Processors or Main Processor Modules (MPMs), two Panel Processors and a Diagnostics Processor (DMPM). An SSI system can operate on two MPMs in the event of the failure of one. It does not need the DMPM to function as an interlocking, as this drives the technician's terminal only.
Data
Geographic interlocking data, relating to the area of railway under control, is installed using EPROMEPROM
An EPROM , or erasable programmable read only memory, is a type of memory chip that retains its data when its power supply is switched off. In other words, it is non-volatile. It is an array of floating-gate transistors individually programmed by an electronic device that supplies higher voltages...
s contained in plug in memory modules. The interlocking program contained in each of the MPMs interprets this data to allow safe passage of trains through its area of control.
External hardware
Railway 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...
and points
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....
are connected to nearby 'trackside functional modules' (TFMs). Each module has a number of outputs and inputs. Each output drives an individual function, such as a signal lamp or an AWS
Automatic Warning System
The Automatic Warning System is a form of limited cab signalling and train protection system introduced in 1956 in the United Kingdom to help train drivers observe and obey signals. It was based on a 1930 system developed by Alfred Ernest Hudd and marketed as the "Strowger-Hudd" system...
inductor. Certain outputs are capable of driving a flashing lamp directly. The inputs are used to send information back to the interlocking, such as indications determined by track circuit
Track circuit
A track circuit is a simple electrical device used to detect the absence of a train on rail tracks, used to inform signallers and control relevant signals.- Principles and operation :...
relay
Relay
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...
s or points detection circuits, for example.
There are two kinds of TFM; the signal module (identified by a red label) and the points module (black label). A maximum of 63 TFMs may be addressed by one SSI interlocking; in practice the number will be limited by timing issues and the need to allow for future expansion.
Data links
Twisted pair
Twisted pair cabling is a type of wiring in which two conductors are twisted together for the purposes of canceling out electromagnetic interference from external sources; for instance, electromagnetic radiation from unshielded twisted pair cables, and crosstalk between neighboring pairs...
copper cable. The data links are duplicated for availability.
A 'data link module' (DLM) is the interface between the data link and the TFMs. A DLM has a blue label.
For transmission over longer distances, fibre-optic cable and pulse-code modulation
Pulse-code modulation
Pulse-code modulation is a method used to digitally represent sampled analog signals. It is the standard form for digital audio in computers and various Blu-ray, Compact Disc and DVD formats, as well as other uses such as digital telephone systems...
may be used. Another type of module, the 'long distance terminal' (LDT) is available for this purpose. An LDT has a gold coloured label.
Market penetration
SSI is widely installed within Great BritainGreat Britain
Great Britain or Britain is an island situated to the northwest of Continental Europe. It is the ninth largest island in the world, and the largest European island, as well as the largest of the British Isles...
, and has some penetration of other Western Europe
Western Europe
Western Europe is a loose term for the collection of countries in the western most region of the European continents, though this definition is context-dependent and carries cultural and political connotations. One definition describes Western Europe as a geographic entity—the region lying in the...
an markets. It was first used at Dingwall
Dingwall
Dingwall is a town and former royal burgh in the Highland council area of Scotland. It has a population of 5,026. It was formerly an east-coast harbor but now lies inland. Dingwall Castle was once the biggest castle north of Stirling. On the town's present-day outskirts lies Tulloch Castle, parts...
in 1984 in connection with RETB
Radio Electronic Token Block
Radio Electronic Token Block is a system of railway signalling used in the United Kingdom. It is a development of the physical token system for controlling traffic on single lines.- How it works :...
signalling. The first conventional SSI scheme was at Leamington Spa
Leamington Spa
Royal Leamington Spa, commonly known as Leamington Spa or Leamington or Leam to locals, is a spa town in central Warwickshire, England. Formerly known as Leamington Priors, its expansion began following the popularisation of the medicinal qualities of its water by Dr Kerr in 1784, and by Dr Lambe...
in 1985. SSI has also been installed in Indonesia, Hong Kong and other countries. Australia is also an extensive user of SSI, particularly New South Wales, where it is installed at busy locations such as Hurstville - Oatley, North Sydney, Wyong, Granville, Enfield, Blacktown, Olympic Park and others within the RailCorp network.
Future
Due to the success of SSI within the UK market, AlstomAlstom
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...
and WRSL
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...
have released products (Smartlock and 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....
respectively) which copy a number of its features. In particular, both re-use the SSI data preparation language and trackside equipment.