Editing Great Western Main Line (section)

===Changes under the Great Western Railway===
The track was supplemented with a third rail for [[dual gauge]] operation, allowing standard gauge {{track gauge|uksg}} trains to also operate on the route, in stages between 1854 and 1875. Dual gauge was introduced as follows: London to [[Reading railway station|Reading]] (October 1861), Reading to {{rws|Didcot}} (December 1856), Didcot to {{rws|Swindon}} (February 1872), Swindon to Thingley Junction, {{rws|Chippenham}} (June 1874), Thingley Junction to {{rws|Bathampton}} (March 1875), Bathampton to Bristol (June 1874), Bristol station area (May 1854). The broad gauge remained in use until 1892, at which point the last 500 miles of track were converted to standard gauge.<ref name = "first hsr2019"/><ref>{{cite book |last = Clinker |first = C. R. |title = New light on the Gauge Conversion |year = 1978 |publisher = Avon-Anglia |location = Bristol, UK |isbn = 0-905466-12-8 |pages = 15–16 }}</ref>

Between 1877 and 1899, the original dual tracks were widened to four in numerous places, mainly in the east half of the line: Paddington to {{rws|Southall}} (October 1877), Southall to {{rws|West Drayton}} (November 1878), West Drayton to {{rws|Slough}} (June 1879), Slough to east side of [[Maidenhead Railway Bridge|Maidenhead Bridge]] (September 1884), Maidenhead Bridge to {{rws|Reading}} (June 1893), Reading station (1899), Reading to {{rws|Pangbourne}} (July 1893), Pangbourne to [[Cholsey railway station|Cholsey and Moulsford]] (June 1894), Cholsey and Moulsford to [[Didcot Parkway railway station|Didcot]] (December 1892); also short sections between Didcot and Swindon, and at Bristol.{{citation needed|date=December 2016}}

Following the [[Slough rail accident]] of 1900, in which five passengers were killed, improved [[vacuum brake|vacuum braking]] systems were used on locomotives and passenger rolling stock; furthermore, [[Automatic train control#United Kingdom|Automatic Train Control]] (ATC) was introduced in 1908.<ref name="Derail">{{cite book |last = Faith |first = Nicholas |date = 2000 |title = Derail: Why Trains Crash |location = London, UK |publisher = [[Channel 4]] |page = 53 |isbn = 9780752271651}}</ref>

Further widenings of the line took place between 1903 and 1910; another round of widening works occurred between 1931 and 1932.{{sfn|Sanderson | Pollard | Thorne | Hradsky |2012| page = 6 }} By the 1930s, trains traversing the GWML were reportedly attaining the highest average speeds in the world.<ref name = "first hsr2019"/>

A legacy of the broad gauge was that trains for some routes could be built slightly wider than was normal in Britain; examples included the 1929-built ''[[GWR Super Saloons|Super Saloons]]'' used on the [[boat train]] services that conveyed transatlantic passengers to London in luxury.<ref>{{cite book |last = Harris |first = Michael |title = Great Western Coaches From 1890 |publisher = David and Charles |year = 1985 |edition = 3rd |location = Newton Abbot |isbn = 0-7153-8050-8 |page = 83}}</ref> When the company celebrated its centenary during 1935, new "Centenary" carriages were built for the Cornish Riviera Express, which again made full use of the wider [[loading gauge]] on that route.<ref>{{harvnb|Harris|1985|page=95}}</ref>

At the outbreak of the [[First World War]] in 1914, the Great Western Railway, and thus the GWML, was taken into government control, as were most major railways in Britain. After the conflict, the companies were reorganised into the [[Big Four (British railway companies)|"big four" companies]], of which the Great Western Railway was one. The railways, including the GWML, returned to direct government control during the [[Second World War]] before being [[nationalisation|nationalised]] to form [[British Rail]]ways (BR) in 1948, thus bringing the line into public ownership.<ref>{{cite magazine |date = January–February 1948 |title = British Railways |magazine = [[The Railway Magazine]] |location = Westminster, UK |publisher = Railway Publishing Company |publication-date = 1 January 1948 |volume = 94 |issue = 573 |page = 72 |language = en-GB}}</ref><ref>{{cite magazine |date = March–April 1948 |title = The Minor British Railways and the Joint Lines |magazine = [[The Railway Magazine]] |location = Westminster, UK |publisher = Railway Publishing Company |publication-date = 1 March 1948 |volume = 94 |issue = 574 |page = 126 |language = en-GB}}</ref>