Western Rail Link to Heathrow

Network Rail plans to spend £5 billion on modernising the Great Western main line, its South Wales branch and other associated lines. The modernisation plans were announced at separate times but their development time-scales, during the second decade of the 21st century, overlap each other. The work includes electrification, resignalling, new and station upgrades. According to Network Rail, the modernisation started in June 2010 and was due to end in 2017. The project has been subject to repeated delays. In November 2016 the government announced that several major elements of the electrification programme would be indefinitely deferred because costs had tripled. The five sections that are delayed are:

Under the Intercity Express Programme (IEP), 21 electric Class 800 trains were ordered as replacements for the ageing InterCity 125 diesels. In May 2016, owing to delays in the modernisation project, it was confirmed that the new trains would be converted to 'bi-mode', meaning they can run on diesel fuel and electric overhead wire.

At the start of the 21st century, the Great Western main line and the Midland Main Line were the last of the major main line routes in the UK using diesel trains as the main source of locomotive power. When the announcement was made in July 2009 to electrify the Great Western (along with the Liverpool-Manchester line), it represented the first big rail electrification project in the UK for 20 years. The South Wales Main Line branch of the GWML is set to be the first electrified cross-country railway line in Wales.

The plan to upgrade the rolling stock on the Great Western was included in the IEP announced in 2007, a Department for Transport ((DfT) led initiative to replace the ageing fleet of InterCity 125 and InterCity 225 train sets currently in use on much of the UK rail network.

In 2009, the only electrified portion of the Great Western was between London Paddington and Airport Junction (west of Hayes and Harlington). This portion of the Great Western is equipped with a 25 kV AC overhead system which was implemented for the Heathrow Express service. The electrification has been extended from Airport Junction to Maidenhead under the Crossrail scheme. Further electrification west of Maidenhead was announced by the DfT separately, though the electrification west of Airport Junction (to take Crossrail to Maidenhead) and west of Maidenhead is likely to be undertaken as one scheme.

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