There are severe disruptions to train services from South Wales to and from London Paddington after damage to electric wires.
There are no trains between London Paddington and Reading following overhead wire damage caused by test train being operated by Hitachi in the Ealing area on Tuesday evening.
Most services will start and terminate at Reading with Great Western Railway services from Wales amongst those affected.
There are also disruptions on the Heathrow Express and TfL rail.
Commuters coming from beyond Reading have been advised to travel to London Waterloo, London Marylebone or London Euston.
Network Rail said engineers are working to fix the damaged lines and recover stranded trains and hope to have two of the four lines working by midday.
But, they have warned of disruptions until at least midday today.
Tickets are being accepted on South Western Railway, Chiltern Railways, Virgin Trains, West Midlands Railway and Transport for Wales - with tickets also being valid tomorrow should people postponing their journey until tomorrow.