Editing Heathrow Express (section)

===Construction===
Construction began in 1993, with an estimated cost of £350 million.<ref name="Railway Technology" /> The principal works were two {{Convert|6.8|km|mi|abbr=in}} single-bore tunnels (including eight escape shafts) and underground stations at {{rws|Heathrow Central}} and [[Heathrow Terminal 4 railway station|Terminal 4]]. [[Railway electrification|Electrification]] of the [[Great Western Main Line]] (GWML) between Paddington and Airport Junction, where the new line diverged from the GWML, was also required to achieve a 15-minute journey time.<ref name="Green-2014a" /> A [[flying junction]] known as the Stockley Flyover was constructed to connect the tunnel to the GWML fast lines. As part of the construction of the project, steps were taken to reduce the environmental and visual impact of the railway, including disguising ventilation shafts as barns.<ref name="BBC News-1998">{{cite news |date=23 June 1998 |title=Heathrow Express takes off |url=http://news.bbc.co.uk/1/hi/uk/118180.stm |url-status=live |archive-url=https://web.archive.org/web/20201220201437/http://news.bbc.co.uk/2/hi/uk_news/118180.stm |archive-date=20 December 2020 |access-date=6 August 2016 |work=BBC News |language=en-GB}}</ref>

In October 1994, a tunnel collapse occurred during construction that led to the [[subsidence]] of a surface building and three large surface craters.<ref name="Harper-2000">{{Cite news |last=Harper |first=Keith |last2= |first2= |date=6 July 2000 |title=Managers 'overlooked risk' in airport tunnel collapse |url=https://www.theguardian.com/uk/2000/jul/06/keithharper |access-date=11 January 2024 |work=The Guardian |language=en-GB |issn=0261-3077}}</ref><ref>{{Cite news |last=Harper |first=Keith |date=16 February 1999 |title=Balfour Beatty fined £1.2m for airport tunnel collapse |url=https://www.theguardian.com/uk/1999/feb/16/keithharper.balfourbeattybusiness |access-date=11 January 2024 |work=The Guardian |language=en-GB |issn=0261-3077}}</ref> The investigation into the collapse led to a six-month delay in opening and additional costs of around £150 million.<ref>{{Cite news |last=Peracha |first=Qasim |date=13 February 2022 |title=A tunnel collapse at Heathrow nearly ruined the airport and a Tube line |url=https://www.mylondon.news/news/nostalgia/how-heathrow-airport-tunnel-collapsed-17579206 |access-date=27 August 2022 |website=MyLondon |language=en-GB}}</ref><ref name="Harper-2000" />