Cotswold Line trains services are currently only able to operate between Oxford and Worcester after Network Rail ordered an emergency closure of a bridge over the River Thames near Culham.
The Nuneham Viaduct, between Culham and Radley, has had a speed restriction over it for a number of years, due to concerns about the condition of its foundations and the adjacent embankments, but the problems worsened late last year.
Network Rail has begun repair work, but last week reduced the speed restriction to 5mph, then on Monday, April 3, engineers ordered the closure of the line to all trains.
Disruption is expected to last until at least the end of Tuesday, April 4.
Passengers travelling to and from London can use a number of alternative routes to travel to and from London, including: the Chiltern Line between Oxford and London Marylebone; West Midlands Railway and Avanti West Coast trains between Worcester, Birmingham and London Euston; and GWR and CrossCountry services between Worcester and Cheltenham to reach London.
Some rail replacement buses are operating between Oxford and Didcot Parkway and GWR tickets are also being accepted on Thames Travel bus routes between Oxford and Didcot and Stagecoach buses between Oxford and Swindon.
The 17.34 train from London Paddington to Hereford is planned to be diverted to run to Worcester via the route through Stroud.
Click here for the latest updates on the situation from GWR Journeycheck.