Mainline passengers are facing severe disruption this morning after damage to overhead electric cables forced the cancellation of all trains into and out of Paddington station.
The issue affects all Great Western Railway (GWR) trains between Paddington and Slough, as well as the Heathrow Express and TfL Rail, which runs the former Heathrow Connect service.
According to the BBC, the 500 metres of damage was caused by a high-speed test train made by Hitachi, which says it is investigating the issue.
Network Rail says it is working to fix the overhead wires and is also attempting to recover trains stranded by the damage.
It warns that lines to and from Paddington may not be open until midday at the earliest, but there is a potential for disruption until the end of the day.
Passengers travelling from beyond Reading into London are advised to take alternative routes to Waterloo, Marylebone or Euston.
Trains will run with a reduced service from Reading to other destinations such as Bristol, Swansea and Exeter, among others.
TfL Rail is still operating a shuttle service between Heathrow Terminals 2, 3 and 4 as usual, as well as between Terminals 2, 3 and 5 on behalf of Heathrow Express.
GWR has reminded passengers that they could be entitled to a refund or compensation if their train has been delayed or cancelled.