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Railway electrification in Great Britain began in the late 19th century. A range of voltages has been used, employing both overhead lines and conductor rails. The two most common systems are 25 kV AC using overhead lines, and the 750 V DC third rail system used in Southeast England and on Merseyrail. As of October 2023, 6,065 kilometres (3,769 mi) (38%) of the British rail network was electrified.[1]
According to Network Rail, as at 2003, 64% of the electrified network used the 25 kV AC overhead system, and 36% used the 660/750 V DC third-rail system.[2]
The electrified network is set to expand over the coming years, as 25 kV electrification is extended to currently unelectrified lines such as the Midland Main Line, as well as lines in the North of England as part of the Northern Hub.[3]
History
Early electrification
The first electric railway in Great Britain was Volk's Electric Railway in Brighton, a pleasure railway, which opened in 1883, still functioning to this day. The London Underground began operating electric services using a fourth rail system in 1890 on the City and South London Railway, now part of the London Underground Northern line. The Liverpool Overhead Railway followed in 1893, being designed from the outset to be electric traction, unlike the City and South London Railway which was designed to be cable hauled initially.
Main line electrification of some suburban lines began in the early years of the 20th century, using a variety of different systems. The Mersey Railway converted to 600 V DC electric multiple-unit operation on 3 May 1903, thus eliminating the problems caused by steam traction in the long tunnel under the River Mersey, and the Lancashire & Yorkshire Railway's Liverpool Exchange to Southport (and on to Crossens) suburban commuter line was similarly electrified at 625 V by March 1904. Both of these lines initially used a fourth rail system.
In 1921, a government committee chose 1,500 V DC overhead to be the national standard,[4] but little implementation followed and many different systems co-existed. During the interwar period, the Southern Railway adopted the 660 V DC third rail system as its standard and greatly expanded this system across its network of lines South of London.
Post-war
After World War II and the nationalisation of the railways in 1948, British Railways (BR) expanded electrification at both 1,500 V DC overhead and 660/750 V third rail. In 1956, BR adopted 25 kV AC overhead as standard for all projects outside logical extensions of third-rail systems.[5]
Twenty-first century
The 25 kV AC network has continued to expand slowly, and large areas of the country outside London are not electrified. In 2007, the government's preferred option was to use diesel trains running on biodiesel, its White Paper Delivering a Sustainable Railway,[6] ruling out large-scale railway electrification for the following five years.
In May 2009, Network Rail launched a consultation on large-scale electrification, potentially to include the Great Western Main Line and Midland Main Line and smaller "in-fill" schemes. Key benefits cited were that electric trains are faster, more reliable and cause less track wear than diesel trains.[7] On 5 June 2009, Lord Adonis was appointed Secretary of State for Transport, and announced the plans to electrify the Great Western Main Line from London as far as Swansea, as well as infill electrification schemes in the North West of England.
In Scotland, where transport is devolved to the Scottish Government, Transport Scotland has extended and continues to expand electrification, for example, on the Airdrie–Bathgate rail link. This is part of a larger plan that has seen many major routes in central Scotland electrified, including the main Edinburgh Waverley–Glasgow Queen Street route. They have pursued electrification with multiple schemes in the Central Belt. All these have been 25 kV AC, as in England and Wales.
In July 2012 the UK government announced £4.2 billion of new electrification schemes, all at 25 kV AC and reconfirmed schemes previously announced by Adonis. These were to be Northern Hub, Great Western Main Line, South Wales Main Line, Midland Main Line, Electric Spine, Crossrail, Gospel Oak to Barking line and West Midlands suburban lines including the Cross-City Line.
On 25 June 2015 the government announced that some of the electrification projects would be delayed or cut back because of rising costs. Electrification work was to be "paused" on the Trans-Pennine route between York and Manchester and on the Midland main line between Bedford and Sheffield. Electrification of the Great Western main line would go ahead but the status of the Reading–Newbury and Didcot–Oxford sections was unclear.[8]
However, in September 2015, the electrification work was "un-paused", but with a delayed completion date.[9] Since then there have been regular updates including one published in October 2016.[10]
On 20 July 2017 Chris Grayling the Secretary of State for Transport cancelled a number of electrification projects citing disruptive works and use of bi-mode technology as an alternative.[11]
Electrification has not been without controversy with cancellations and various appearances of the Secretary of State for Transport called before the Transport Select Committee. The Transport Select Committee published its report into various matters including regional investment disparity on the railways and calling again for the reinstatement of various cancelled electrification schemes.[12]
A written question was submitted and answered in parliament regarding route miles electrified in the years 1997–2019.[13]
In March 2019 the Railway Industry Association published a paper on Electrification cost challenge suggesting ways forward and a rolling program of electrification.[14]
Future of third rail
In June 2011 Peter Dearman of Network Rail suggested that the third-rail network will need to be converted into overhead lines. He stated: "Although the top speed is 100 mph (160 km/h), the trains cannot go over 80 mph (130 km/h) well and 25% of power is lost from heat." Agreeing that conversion would be expensive, he said that the third rail network is at the limit of its power capability, especially as trains become more advanced in technology.[15] The July 2012 Department for Transport High Level Output Specification for Network Rail Control Period 5 includes the conversion of the South West Main Line between Southampton Central and Basingstoke from 750 V DC third rail to 25 kV AC overhead as part of a scheme to improve rail freight capacity from Southampton Port. This conversion would be a pilot scheme to develop a business case for full conversion of the third-rail network.[16] The Office of Rail and Road (ORR) has also stated that, on safety grounds, third-rail 750 V DC has a limited future.[17]
Existing systems – overhead line (OHL)
National Rail: 25 kV, 50 Hz AC overhead
British Railways chose this as the national standard for future electrification projects outside of the third rail area in 1956. Following this, a number of lines that were originally electrified at a different voltage were converted, and a number of lines have been newly electrified with this system. Work started in the late 1950s. The first major electrification project using 25 kV was the West Coast Main Line (1959–1974). Initially this was Crewe, Manchester and Liverpool south into London and Birmingham. Weaver Junction north to Glasgow followed later. The 25 kV network has been gradually expanded ever since:
Existing
- Electrified from London Paddington via Reading, Swindon and Bristol Parkway to Cardiff Central.[18]
- Electrified from Reading to Newbury.[19]
- Electrified from London Paddington to Heathrow Terminal 4 in 1994 in a joint venture between British Rail and the British Airports Authority using the Mark 3B series.
- Electrified from London Euston during the late 1950s and mid-1960s using the Mark 1 series under the BR 1955 Modernisation Plan to Crewe, extended to Glasgow Central in 1974 using the Mark 3A range.
- Northampton: see Northampton loop.
- Birmingham New Street: see Rugby–Birmingham–Stafford line.
- Crewe to Liverpool Lime Street.
- Stafford to Manchester Piccadilly: see Stafford–Manchester line and Crewe–Manchester line.
- The "Abbey Flyer" (Abbey Line) was electrified in 1987–88 by Network SouthEast.
- Edinburgh Waverley in 1989 (from Carstairs Junction in conjunction with East Coast Main Line electrification)
- In 2003, the Crewe–Kidsgrove section of the Crewe–Derby line was electrified as a diversionary route for the WCML.
- Since 1999, the line has been modernised and the overhead line equipment has been refurbished and renewed from Mark 1 / Mark 3A to UK1 range to allow an increase line speeds from 110 mph to 125 mph (with 140 mph capability in areas previously fitted with Automatically Tensioned Mark 1 equipment - subject to upgrading of the balance weight arrangement to provide individually tensioned contact / catenary wires and regrading of the contact wires). At the same time sections of the line are being progressively changed to autotransformer system.
- Electrified between London St Pancras and Bedford in 1983 using the Mark 3B range, and Dock Junction to Moorgate - now cut back to City Thameslink.
- Electrification from Bedford to Kettering and Corby using the UK Master Series (MS125) range is expected by Spring 2021 (MML Phase 1), further extensions to Leicester, Nottingham Trent Junction and Sheffield (via Derby) by 2023 (MML Phase 2) were cancelled in July 2017. In November 2021, the Integrated Rail Plan (IRP) was published.[20] This included full Midland Main Line electrification. On 21 December 2021 it was announced that work would start immediately on electrification of the section between Kettering and Market Harborough.[21][22][23] Grant Shapps claimed this work was proof the IRP was being implemented quickly but was met with ridicule.[24]
- Newest main line, completed in 2007. Links London St Pancras with Ashford International and the Channel Tunnel.
- Electrified in two parts: 1975–78, and 1984–91
- The line between London King's Cross and Royston was electrified between 1976 and 1978 using the Mark 3A range as part of the Great Northern Suburban Electrification Project. This included the Hertford loop line. The section between Royston and Cambridge was electrified in 1988 using the Mark 3B range.[25]
- In 1984, authority was given to electrify to Edinburgh and Leeds.[26] The section between Hitchin and Peterborough was completed in 1987, and Doncaster and York were reached in 1989. By 1990, electrification had reached Newcastle, and in 1991 Edinburgh Waverley. The Mark 3B range was used throughout the electrification scheme, certain areas are presently being upgraded to the Mark 3D design range, this will eliminate known corrosion issues with the AWAC catenary and replace solid stainless steel droppers with flexible copper current carrying designs. Some headspan to portal conversions are also taking place.
- In order to keep construction teams working, two additional schemes were authorised, to Carstairs and North Berwick (North Berwick Line).
- At the peak of the electrification project during the late 1980s, it was claimed to be the "longest construction site in the world" at over 250 miles (400 km).
This covers the lines from London Liverpool Street (Bethnal Green Junction) to Chingford, Enfield Town, Hertford East and Cambridge. In the 1960s, the lines to Chingford, Enfield Town and Cheshunt were electrified at 6.25 kV, from Cheshunt to Bishop's Stortford and Hertford East at 25 kV. The Lea Valley line between Coppermill Junction and Cheshunt was electrified at 25 kV in 1969. All the 6.25 kV areas were converted to 25 kV in 1983. In 1987, electrification was extended from Bishop's Stortford to Cambridge at 25 kV. In 1990 the line to Stansted Airport opened, and in 1992 electrification was extended from Cambridge to King's Lynn along the Fen Line.
- London Liverpool Street to Norwich.
- Converted from 1,500 V DC (see 1,500 V DC section "Shenfield Metro")
Converted from 6.25 kV/1,500 V DC to a combination of AT and FT 25 kV Mark GE (Great Eastern) between 1976 and 1980. Presently being upgraded to the GEFF (Great Eastern Furrer + Frey) range altering the catenary from a compound to simple sagged arrangement.
- Romford–Upminster line
- Shenfield–Southend line
- Crouch Valley line
- Braintree branch line
- Mayflower line
- Sunshine Coast Line
London Fenchurch Street to Shoeburyness. The majority was originally electrified at 6.25 kV, final sections converted to 25 kV in March 1989.
Local lines within London electrified with 25 kV are:
- North London line, between Acton Central and Stratford.
- Lea Valley lines
- Gospel Oak to Barking line
- Various other suburban lines in the north of the city are electrified as part of other routes mentioned above.
- West Midlands
- West Coast Main Line routes electrified in the 1960s:
- Commuter lines out of Birmingham New Street:
- Cross-City Line: electrified 1993
- Chase Line: New Street to Rugeley Trent Valley completed 2017
- Manchester and North West area
- Manchester to Glossop / Hadfield (converted from the truncated 1500 V DC Manchester-Sheffield-Wath electric railway)
- Manchester to Liverpool via Earlestown line: electrified in 2015 as part of the Northern Hub project.
- Manchester, South Junction and Altrincham Railway (part was converted to Manchester Metrolink)
- Styal Line: including branch to Manchester Airport
- Manchester–Preston line: via Bolton and Chorley completed 2019
- Preston to Blackpool North: completed 2018
- Stafford–Manchester line: branch of the WCML, electrified in the wake of the BR 1955 Modernisation Plan
- Crewe–Manchester line: branch of the WCML, electrified in the wake of the 1955 Modernisation Plan
- Leeds area
In 1994, a project to electrify some of the local lines around Leeds was given authority to proceed. The project was called the "Leeds North West Electrification", which electrified:
- Airedale line to Skipton and Bradford Forster Square
- Wharfedale line to Ilkley
- Wakefield line electrified in 1989 as part of the East Coast Main Line electrification to London King's Cross
- In 2020 the electrification of the first part of the stalled TransPennine project, from Leeds to Dewsbury and Huddersfield, was approved and work also commenced on the York to Church Fenton section of the York to Leeds line.
- Edinburgh
- In 1991, the ECML to Edinburgh was electrified. A few local routes were also electrified.
- Edinburgh Crossrail: Edinburgh Waverley to Newcraighall. The service is by DMUs, pending reopening of part of the Waverley Route.
- North Berwick Line: Edinburgh Waverley to North Berwick
- Glasgow–Edinburgh via Carstairs line: some North Berwick Line trains continue to Glasgow Central. Intercity trains from the ECML continue to Glasgow Central.
- Central Scotland
The route from Edinburgh to Glasgow via Bathgate has been reinstated between Bathgate and Airdrie and electrified throughout. It opened on 11 December 2010. The electrification of the main inter-city route between Edinburgh and Glasgow Queen Street High Level via Falkirk was completed in 2017. The project, known as the Edinburgh to Glasgow Improvement Programme, entailed infill electrification in the Glasgow area and Greenhill Junction to Stirling, Dunblane and Alloa, which mainly carry commuter services. Electric services on these lines commenced in December 2018.
- Glasgow Suburban
Suburban electrification was begun during the 1960s in the wake of the BR 1955 Modernisation Plan. Electrification was piecemeal and is still incomplete, with a few commuter lines still unelectrified such as the East Kilbride branch and Glasgow to Anniesland via Maryhill, and the mainline from Glasgow to Carlisle via Kilmarnock and Dumfries.
The Glasgow Suburban railway network can be divided into three main areas:
- North Clyde Line: also known as the "Glasgow North Electric Suburban Line", one of the first lines in Glasgow electrified in 1960 (Helensburgh Central, Balloch and Milngavie to Glasgow Queen Street (Low Level) and to Springburn and Airdrie).
- South Clyde: the Cathcart Circle Line (Glasgow Central to Newton and Neilston) was electrified on 22 May 1962.[27] The Inverclyde Line (Glasgow Central to Gourock and Wemyss Bay) was electrified in 1967.[27] The Ayrshire Coast Line (Glasgow Central to Ayr, Largs and Ardrossan Harbour) was electrified in 1986–1987.[28] The Paisley Canal line was electrified to Corkerhill from Glasgow Central, in late 2012 extended to Paisley.
- Argyle Line: between Dalmuir and Milngavie via Glasgow Central (Low Level) to Hamilton Circle, Larkhall, Lanark and Carstairs (via Hamilton, Motherwell or Holytown). There is also peak service to Coatbridge Central.
On the Glasgow–Edinburgh via Carstairs line, some North Berwick Line trains continue to Glasgow Central. A single daily East Coast intercity train from the ECML continues to and from Glasgow Central. The Shotts Line, Holytown Junction to Kirknewton was electrified in April 2019. The Cumbernauld Line to Springburn and the remaining section of the Motherwell–Cumbernauld line was electrified in mid 2014. The line between Springburn and Glasgow Queen Street (High Level) has not yet been completed. Until Glasgow Queen Street High Level has been electrified, electric Cumbernauld Line trains reverse at Springburn and run through Glasgow Queen Street Low Level station. The Whifflet Line between Whifflet and Rutherglen via Carmyle was electrified in late 2014.
2010s Network Rail electrification programme
In 2009, Lord Adonis was appointed Secretary of State for Transport. After a gap of more than a decade, electrification was back on the agenda and Adonis announced plans to electrify the Great Western Main Line from London to Swansea, as well as infill electrification schemes in the North West of England. In July 2012, the UK government announced £4.2 billion of new electrification schemes, all at 25 kV AC and reconfirmed schemes previously announced by Adonis. These were to be Northern Hub, Great Western Main Line, South Wales Main Line, Midland Main Line, Electric Spine, Crossrail, Gospel Oak to Barking line and West Midlands suburban lines. Rail transport in Scotland is a devolved matter for the Scottish Government but they too have pursued electrification with multiple schemes in the Central Belt. All these have been 25 kV AC also as in England and Wales. Electrification has not been without controversy with cancellations and various appearances of the Secretary of State for Transport called before the Transport Select Committee. The number of route miles electrified in these years was answered to a written question in parliament.[13]
In November 2019 the annual statistics for route miles electrified was published by the DfT and shows that 38% of the UK network is now electrified.[29]
The projects have been subject to cost overruns and delays, and on 8 November 2016 the government announced that several elements of the Great Western Main Line electrification programme would be indefinitely deferred.[30] In an attempt to mitigate and improve the cost situation the Railway Industry Association published a report in March 2019 detailing why costs had risen and suggested ways forward.[14]
However, in the new parliament after the 2019 General election, the Transport Select Committee chaired by Huw Merriman has met on a number of occasions and continued the "Trains fit for the future" enquiry theme started by the previous committee. On 23 March 2021, after many witnesses were called and written and oral evidence considered, a report was released calling for an immediate resumption of electrification in a rolling programme.[31] However, in December 2021 in a story that appeared in the Telegraph it was stated that the Treasury had declined to support the electrification programme.[32][33] Reputable peer reviewed journals state that electrification is the most relevant technology for reducing transports effect on the environment.[34]
Other systems
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1,500 V DC, overhead
- Tyne and Wear Metro: The Tyne & Wear Metro, which opened in 1980, is now the only system left in the UK using the 1500 V DC overhead lines. Although it is often described as "light rail", it is closer to a heavy metro, using only segregated track. Much of its route follows that of the previous Tyneside Electrics, which had been converted to diesel by 1967. Since 2002, the Metro has shared main-line track on the Durham Coast Line to Sunderland. This presents a potential problem for main-line services if routes into Sunderland or Newcastle upon Tyne that use this section were to be electrified at 25 kV AC.
Historically, there were more lines electrified at 1,500 V DC, but these have all since been either converted to 25 kV AC or closed. (see 1,500 V DC, overhead (historic))
750 V DC, overhead
Used on several tram systems:
- Edinburgh Trams
- Manchester Metrolink
- Sheffield Supertram
- Croydon Tramlink
- Nottingham Express Transit
- West Midlands Metro
Other overhead systems
- Blackpool Tramway: originally 550 V DC, in 2011 upgraded to 600 V to operate more modern rolling stock.
- The National Tramway Museum at Crich, Derbyshire uses 600 V DC. This voltage was chosen for maximum compatibility with its historic fleet of trams as well as more modern units.
- The Wirral Tramway uses 550 V DC.[35]
- The Seaton Tramway uses 120 V DC.
Existing systems - third and fourth rails
National Rail: 650 V - 750 V DC, third rail (top contact)
- Southern Electric
The extensive southern third rail electric network covers South London and the southern counties of Hampshire, West Sussex, East Sussex, Surrey and Kent and Dorset,
The London and South Western Railway (L&SWR) third-rail system at 660 V DC began before World War I from London Waterloo to suburban destinations. The Southern Railway was formed in the 1923 grouping; it adopted the L&SWR system, and by 1929 the London, Brighton and South Coast Railway (LB&SCR) suburban overhead network was replaced by third rail. The South Eastern Main Line was electrified at 600 V, later upgraded to 750 V DC. The third rail extended throughout most South London lines out of all its London termini. Throughout the 1930s, there was much main line electrification, including the Brighton Main Line (including East, West Coastways and related routes in 1932–1933), the Portsmouth Direct line (4 July 1937) and to Maidstone and Gillingham (1939).
After World War II, electrification was soon resumed in the newly nationalised British Railways' Southern Region. The BR 1955 Modernisation Plan included the two-stage "Kent Coast Electrification". The Chatham Main Line was completed, followed by the South Eastern Main Line and related lines. The voltage was raised from 660 V to 750 V.[36] Since then, all electrification has used 750 V; lines electrified before then remain at 660 V. Attention then switched to the neglected former L&SWR area (then the South Western Division). The South West Main Line (SWML) to Southampton Central and Bournemouth was electrified in 1967 and to Weymouth in 1988.
During sectorisation in the 1980s, Network SouthEast conducted extensive infill electrification. The Snow Hill tunnel was reopened, enabling Thameslink. The Hastings Line, Eastleigh–Fareham line and the Oxted line (East Grinstead branch) were electrified. This left only a few lines unelectrified: the West of England line, the Wessex Main Line, the North Downs Line, the Oxted line (Uckfield branch), the Marshlink line and the Eastleigh–Romsey line.
Two lines of the Merseyrail network; the Northern line and the Wirral line use 750 V DC third rail[37][38] (see Suburban electrification of the London, Midland and Scottish Railway for its history).
The single remaining national rail line on the Isle of Wight, from Ryde Pierhead to Shanklin (with the Wroxall to Ventnor section closed), was electrified in 1967, so that former London Underground rolling stock could be used, due to the limited height of Ryde Tunnel. The Island Line used 660 V DC third rail,[39] as it was a cheaper option to convert the LUL stock into third rail, and implement third rail only on the line. The rolling stock currently used is British Rail Class 484s (D-Train). The line was upgraded to a 750 V DC third rail system in 2021 to allow Class 484 units to be used.[40]
- Euston to Watford Junction (Watford DC line).
- Richmond to Stratford (North London line). 750 V DC third rail from Richmond to Acton Central.
- West London line. 750 V DC from near the location of the former St. Quintin Park & Wormwood Scrubs railway station to Clapham Junction (shared with Southern services).
- East London line. Highbury & Islington to New Cross station and the junctions with the South London network near New Cross Gate station and Queens Road Peckham station. Formerly, the East London Line was a much shorter London Underground line with fourth rail 630 V DC between Shoreditch (closed 2006) and New Cross/New Cross Gate.
See Suburban electrification of the London, Midland and Scottish Railway for Euston–Watford DC Line history.[41]
In 1970, the North London DC lines and the Class 501 EMUs used on these services were converted for third-rail operation, with the fourth rail generally being removed on sections not used by London Underground (LUL). Some fourth rail was retained in the Gunnersbury and Queens Park areas for emergency use by LUL. With the closure of Broad Street, the North London line was joined with the Stratford to North Woolwich line; this was electrified with third rail and overhead line as far as Stratford, third rail to North Woolwich. Two branches of the Watford DC line have been closed: to Rickmansworth in 1952 (to passengers, to goods in 1967) and to Croxley Green in 1996.
The Watford DC line between Queen's Park and Harrow & Wealdstone and the North London Line between Richmond and Gunnersbury are used by London Overground trains designed for 750 V third rail and Bakerloo line trains designed for 630 V third and fourth rail. As a compromise, the nominal line voltage is 650 V, and since 1970 the centre rail has been bonded to the return running rail.[42] There are no special provisions required at Queens Park, where the two dissimilar systems meet, just a gap longer than one coach of a Bakerloo line train at the entry to (and exit from) the Bakerloo, which operates with a nominal -210 V on the fourth rail and +420 V on the third rail. There is no bridging of the incompatible systems as trains pass from one to the other since, like all UK electric trains intended to run extensively in tunnels, there is no continuity of traction power circuits between vehicles of the train.
A similar arrangement applies between Putney Bridge and Wimbledon, where the District line runs over tracks owned by Network Rail, which is also used by South Western Railway, though normally only for stock movements.
The Northern City Line connects the East Coast Main Line to Moorgate. It was isolated by the abandonment of the 1930s New Works Programme (and the development of the Metropolitan Green Belt). Tube services were truncated at its northern end by the Victoria line in 1964 at Drayton Park. The remainder was handed over to British Rail in 1975 in conjunction with the suburban electrification of the East Coast Main Line. The line uses third-rail DC electrification between Moorgate and Drayton Park, where trains switch to 25 kV AC overhead.
630 V DC, fourth rail (top contact)
The London Underground is a large metro system operating across Greater London and beyond, commonly known as "the Tube". Its 408-kilometre (254 mi)[43] is made up of 11 lines; electrification began during the 1890s. It was largely unified between 1900 and 1910 and nationalised in 1933, becoming the railway component of London Transport (LT). A major expansion programme (the "New Works") was launched, in which LT took over several urban branches of mainline railways.
The Underground is mostly in North London; its expansion into south London was limited by geology unfavourable to tunnelling and by the extensive main-line network, much of which was being electrified (see "Southern Electric"). The Underground uses a relatively uncommon four rail system of electrification. Two standard gauge rails are the running rails; the outer third rail carries positive current at +420 V DC and the inner fourth rail is the negative return at –210 V DC, giving a supply voltage of 630 V DC. The chief advantage of the fourth-rail system is that, in tunnels with a metallic (usually cast-iron) lining, the return traction current does not leak into the lining causing electrolytic corrosion there or in adjacent utility mains. It also means that the two running rails are available exclusively for track circuits.
The surface sections use the fourth rail solely for operational consistency: the system shares track with Network Rail in several places. Where the track is shared with 750 V third-rail stock, the central rail is bonded to the running rails and the outside rail electrified at 660 V. This allows both types of train to operate satisfactorily. The suburban network of the London & North Western Railway (LNWR) was electrified in co-operation with the Underground, but during the 1970s British Rail introduced third-rail EMUs and the sections of the LNWR suburban network not used by the Underground had the fourth rail removed (see "London and North Western Railway", above).
The Underground has carried out studies to consider raising the voltage above the present 630 V nominal.[44] New equipment at their substations does allow for a future increase to a standard 750 V nominal. In addition, the electrical equipment of new trains are also based on the use of 750 V rated equipment. So, whilst new equipment is being designed to for 750 V operation, no decision to increase the voltage has yet been made public by the Underground.
As part of the Four Lines Modernisation project, the subsurface lines have been upgraded 750v DC 4th rail operation which is supported by the newer S7 and S8 stock.[45] Where S-stock trains regularly interline with deep-level tube stock or where power supplies are shared, the voltage has been kept at 630v DC since Jubilee line and the present day Piccadilly line rolling stock are incompatible with the higher voltage. As of February 2024, the only parts of the subsurface network which remain at a nominal 630v are: between Finchley Road and Harrow-on-the-Hill (where the alignment and supply is shared with the Jubilee line), between Finchley Road and Uxbridge (where tracks are shared with the Piccadilly Line) and between Baron's Court and Ealing Broadway (where tracks are shared with the Piccadilly Line) but not on the Richmond and Wimbledon branches which have been upgraded to 750v operation.[46] The upgraded system allows for regenerative braking and the associated voltage surges up to 890v on the higher voltage tracks and up to 790v or 650v on the remaining 630v tracks.[46]
750 V DC, third rail (bottom contact)
This uses bottom-contact composite third rail, with an aluminium body and a steel contact surface. The advantage of this is a low-resistance, high-current-capacity rail with a durable steel surface for current collection. The rail may be surrounded by insulating material on the top and sides to reduce the risk of electrocution to railway staff and trespassers. The bottom-contact system is less prone to derangement by snow than top contact.
750 V DC, fourth rail (top contact)
This system is unique to this line of London Underground operated railways. The use of 750 volts came about because the line was originally owned by Railtrack and operated by Network South East. It was upgraded in 1992/3 for both traction supply and rolling stock. Railtrack upgraded the original three rail system to four rail to solve problems with electrolytic damage to the iron tunnel linings (the reason four rail operation was adopted for all other tube lines). They also changed the voltage to 750 volts which had been adopted as their standard DC operating voltage some years earlier. The line was subsequently sold to London Underground in 1994 who inherited the non standard system (for London Underground). The line is still powered from its own substation located in the Waterloo depot.
As part of Four Lines Modernisation project, most of the subsurface network also operates at 750v DC fourth rail.[45]
600 V DC, third rail (top contact)
- Glasgow Subway, electrified in 1935
250 V DC, third rail (top contact)
- Hythe Pier Railway, electrified in 1922
110 V DC, third rail (top contact)
- Volk's Electric Railway was originally electrified at 50 V DC, raised to 160 V in 1884 and reduced to 110 V DC during the 1980s.
100 V DC, four rail
- The elevated "monorail" at the National Motor Museum, Beaulieu uses rubber tyres running on two metal tracks, one on either side of the central guide. Because it is rubber-tyred, it requires two current conductors and two collectors (hence the four-rail designation).
Electrical Control
Mainline network
All electrified railways require Electrical Control Rooms (ECR) to administer the supply of traction current and respond to fault conditions or emergency incidents. Until 2016, electrical control was carried out by 13 standalone ECRs constructed during the British Rail era.[47] However, it is now intended that electrical control will be transferred to 8 of the 12 Rail Operating Centres (ROC) along with all signalling and train control in general.[48] In addition, the Crossrail and South Wales Metro projects have introduced new electrical control rooms onto the heavy rail network which are part of those systems' respective Route/Rail Control Centres (RCC).
Including the legacy standalone ECRs (all of which are still operational to differing degrees); the new ECRs that have been commissioned as part of ROCs; smaller systems' RCCs; and HS1's control centre at Ashford, the mainline network's electrification is currently controlled at 20 locations.