Network SouthEast was a British Rail Sector formed on 10th June 1986 when the London and South East Sector was rebranded. Sectors were formed when the six passenger regions of British Rail (Southern Region of British Railways, Western Region of British Railways, London Midland Region of British Railways, Eastern Region of British Railways (merged with North Eastern in 1967), North Eastern Region of British Railways (merged with Eastern in 1967), Anglian Region of British Railways (formed in 1988) and Scottish Region of British Railways) were reorganised into business led-sectors. This process known as 'Sectorisation' saw BR move from its embedded regional structure into three business-led passenger sectors. The other two passenger sectors were; InterCity and Provincial. InterCity was viewed as a 'commercial railway' and London & South East and Provincial as 'social railways'. The sectors of British Rail were merely a business and planning units rather than an operational or administrative one. This meant the regions maintained control over the day to day running of trains were as the sectors specified the nature and quality of service to be provided, prepared investment projects, developed new products and marketing initiatives, and took profit responsibility for the services The rebranding of the London and South East to Network Southeast included repainting of trains and stations in a hope of reinvigorating the sector. With the Organisation for Quality in 1992 the regions of British Rail were abolished and the sector took over Civil Engineering and the running of trains, under the Network SouthEast 2000 program, also known as Network 2000. Under this program the six individual divisions of the sector became nine profit centres responsible for the running of trains on individual routes. OfQ was overseen by John Welsby a former economist for the Department of Transport who had become Chief Executive of BR in 1990. Network SouthEast was run from Network House, 1 Eversholt Street, London, NW1 1DN, next to Euston station. The sector was initially headed by Chris Green who was then replaced by John Nelson on 6th January 1992 when the former moved to head InterCity. Network SouthEast was the largest of the business sectors as well as being the 15th largest company in the United Kingdom, in terms of turnover. Initially the Area Managers of each Region controlled the track, stations and rolling stock but after the abolition of the Regions, under the Organisation for Quality, Network SouthEast became responsible for 2,017 miles of track, 942 stations and 7,160 rolling stock vehicles. It ran trains across the South East of England as well as commuter trains into all of the London terminuses from East Anglia, the Midlands and along the Great Western Mainline. This was spilt over six divisions: North, East, West, Southwest, Southcentral and Southeast, within which the regions operated trains through 27 administrative areas. These were latter split into the nine profit centres used to run trains under the Network 2000 program. These were split into two groups, the South Group: Thames Link, South East, South Central, South West, and the North Group: Great Eastern, London Tilbury and Southend, West Anglia and Great Northern, North and Thames and Chiltern. Each of these was headed by a Divisional Director who reported to the Group Director who in turn reported to the Sector Director. In addition to this the sector included the only Underground stock to be run by British Rail, the Waterloo and City line as well as on the Isle of Wight, part of the South West division. These profit centres were broken up, in 1994, into the 11 Train Operating Units, Great Eastern, London Tilbury Southend Rail, North London Railways, Network South Central, South Eastern, South West Trains, Isle of Wight, Chiltern Lines, Thames Trains, Thames Link and West Anglia and Great Northern. Network SouthEast ceased to operate on 1st April 1994 when it was split into 11 franchises operated directly by the British Railways Board until they were sold off between 1995 and 1997.

The British Railways Board was an independent statutory corporation responsible for running the British railway network from 1963. It was established by the Transport Act 1962, which abolished the British Transport Commission and divided its undertakings between five newly-created bodies: the British Railways Board, the British Waterways Board, the British Transport Docks Board, the London Transport Board, and a Transport Holding Company. The British Railways Board was responsible for running the railway network, as well as managing government-owned railway hotels. Members of the British Railways Board were also appointed by the Minister for Transport. The first Chair of the British Railways Board was Dr Richard Beeching. The British Railways Board operated through regional boards, which were responsible for regional sections of the railway network. These regions were Southern, Western, London Midland, London and North Eastern, Eastern, and Scottish. Members of these regional boards were appointed by the British Railways Board, in consultation with the Minister for Transport. The British Railways Board also operated a series of committees to manage every aspect of railway control, including committees for finance, technical, works and property. These committees were frequently reorganised throughout the life of the British Railways Board, under both different Chairs of the Board and different governments. Several changes occurred during the 1960s. The Board had two new Chairs; Stanley Raymond, who replaced Richard Beeching in 1965, and his successor Henry Johnson, who became Chair in 1967. In 1968, the Transport Act transferred the control of the Sundries and Freightliner divisions from the British Railways Board to National Carriers Ltd and Freightliners Ltd. The Board retained a forty-nine per cent stake in Freightliners Ltd. During the 1970s, the British Railways Board created several subsidiary companies which were to manage some of its undertakings. These included British Transport Hotels Ltd, British Rail Engineering Ltd, and British Rail Hovercraft Ltd. Many of these subsidiary companies were sold under the Conservative governments of the 1980s. There were also two new Chairs during this time. Richard Marsh replaced Henry Johnson in 1971, and Peter Parker became chair in 1976. The privatisation of the British rail network during the 1990s radically changed the role of the British Railways Board. The Transport Act 1993 established Railtrack, a publicly-owned company. The Act transferred the ownership of track and railway infrastructure from the British Railways Board to Railtrack, in addition to the control of signals. Railtrack also replaced the British Railways Board as the body responsible for track investment and maintenance. The British Railways Board remained in existence after these changes, but only performed residual functions relating to pensions, liabilities, and non-operational railway land. The Board also continued to operate the British Transport Police service. The British Railways Board was abolished by the Transport Act 2000, which transferred the remaining functions of the Board to the newly-created Strategic Rail Authority.

GEC Alsthom Traction was formed from the merger of GEC and Alcatel-Alsthom (formerly known as Compagnie Générale d’Electricité of France) in 1989, which created a Joint Venture of their Power Engineering businesses (Power Stations, Switchgear and Rail Transport). The UK headquarters of the traction business was based at Trafford Park in Manchester. GEC Alsthom purchased train builder Metro-Cammell based at Washwood Heath, Birmingham from the Laird Group in 1989, bringing into the group a company which had a long association with GEC Traction and its predecessors as a vehicle builder. GEC Alsthom Traction became part of the Transport Equipment Group (TEG) with the French sites Ornans (motors), Tarbes (control equipment) and Villeurbanne (electronic systems, converters and control equipment), later joined by ACEC Transport (Charleroi). A major project in this period was the design and build of the Trans Manche Super Train (TMST) for London. TMST, later known as Eurostar Class 373, was the first TGV with asynchronous traction drive, all TGVs until later Duplex fleets having DC or synchronous traction drives. It was designed for three voltages 750v DC for the UK Southern Region, 3000v DC for Belgium and 25kV for the tunnel and France, and later for HS1 high speed line from the tunnel to London. SNCF later added 1500v DC to 4 trains to allow the trains to run to the Alps. In 1993, as part of the rationalisation of the equipment group, it was announced that the headquarters of GEC Alsthom Traction would move to Preston, leaving only Power Module design and manufacture at Manchester, and a project team was set up at Preston for the regeneration of the Preston site, with the eventual closing of all activities at Trafford Park in 1998. At the same time, there was rationalisation of product lines in the equipment group, with inductors and auxiliary converters moving to Preston, traction motor design concentrated at Ornans, switchgear at Tarbes and electronic control systems at Villeurbanne. GEC Alsthom Traction became Alstom Traction on 22nd June 1998 and the company name survived to 19th August 2008 but was increasingly integrated within the Transport division of Alstom.

In 1988 a joint-venture was formed between the General Electric Company (GEC) with Compagnie Générale d’Electricité (CGE) that led to the establishment of an Anglo-French company, GEC Alsthom in 1989. This company encompassed the Power Generation and Transmission, Rail Transport (25%) businesses from the constituent companies of GEC and CGE. The GEC Alsthom company headquarters were based at Rugby, Warwickshire and the headquarters for Transport were based at Trafford Park in Manchester. GEC Alsthom purchased train builder Metro-Cammell based at Washwood Heath, Birmingham from the Laird Group in 1989, bringing into the group a company which had a long association with GEC Traction and its predecessors as a vehicle builder. In 1998 GEC Alsthom was re-formed and became Alstom with Transport / Alstom Transport one of its major divisions.