Great Western Main Line

The Great Western Main Line (GWML) is a main line railway in England that runs westwards from London Paddington to Bristol Temple Meads. It connects to other main lines such as those from Reading to Penzance and Swindon to Swansea. The GWML is presently a part of the national rail system managed by Network Rail while the majority of passenger services upon it are provided by the current Great Western Railway franchise.

The GWML was built by the original Great Western Railway company between 1838 and 1841, as a dual track line in the 7 ft (2,134 mm) broad gauge. The broad gauge remained in use until 1892, after which standard gauge track has been exclusively used. Between 1877 and 1932, many sections of the GWML were widened to four tracks. During 1908, Automatic Train Control (ATC) was introduced as a safety measure. In 1948, the Great Western Railway, and thus the GWML, was merged into the Western Region of British Railways.

During the 1970s, the GWML was upgraded to support higher line speeds, as a result of which many sections permitted 125 mph (201 km/h) operations, enabling the newly-introduced InterCity 125 high speed train (HST) to make faster journeys. British Rail proposed widespread electrification of the line in the late 1970s, although this was not speedily implemented. During the mid 1990s, a stretch of the GWML between London Paddington and Hayes & Harlington was electrified using 25 kV AC overhead lines for the Heathrow Express. Further, although not total, electrification was carried out during the 2010s; this permitted the replacement of diesel-powered trains such as the InterCity 125 and Class 180 with electric and bi-mode train sets such as the Hitachi Super Express high speed trains, specifically the Class 800s and Class 802s. Due to budget overruns, the British government deferred electrification of the section through Bath from Royal Wootton Bassett to Bristol in 2016.

Communities served by the GWML include West London (including Acton, Ealing, Hanwell, Southall, Hayes, Harlington and West Drayton); Iver; Langley; Slough; Burnham; Taplow; Maidenhead; Twyford; Reading; Tilehurst; Pangbourne; Goring-on-Thames; Streatley; Cholsey; Didcot; Swindon; Chippenham; Bath; Keynsham; and Bristol. The route includes dozens of listed buildings and structures, including tunnel portals, bridges and viaducts, stations, and associated hotels. Presently, the GWML is electrified between London Paddington and Royal Wootton Bassett. In the long term, Network Rail plans to install European Rail Traffic Management System (ERTMS) in-cab signalling across the entire line.