the line was electrified from paddington to heathrow airport in the late 1990s. work to electrify the rest of the route started in 2011. the aim was to complete the work all the way to bristol by 2016. parts of the programme have been deferred because costs have tripled. the four sections deferred are: didcot to oxford, bristol parkway to bristol temple meads, royal wootton bassett junction to bristol temple meads and the thames valley branches to henley-on-thames and windsor.