Dozens of trains have been cancelled after signalling issues led to a freight train blocking a line near Avonmouth.
Disruption on the route between Bristol Temple Meads and Severn Beach is expected until at least 12:00 BST, with no service running between Clifton Down and Severn Beach.
Rail replacement buses in the form of 16-seater minibuses have been put in place but these will not run to the train timetable due to the extended journey time by road, Great Western Railway said.
Train tickets are also being accepted on a number of local bus services at no extra cost.