Rail passengers face a second day of disruption today after cracks were found on high-speed trains.
Great Western Railway, London North Eastern Railway, Hull Trains and TransPennine Express were all affected on Saturday.
GWR and LNER both advised people not to travel after Hitachi 800 trains had to be pulled from lines.
GWR operates 93 trains of that model and urged customers to apply for refunds, warning disruption could continue "into the following week".
The issue is affecting GWR trains between Paddington and Bristol, Swansea, Penzance, Hereford and Cheltenham Spa.
Suburban and rural services which are still running will be "very busy" as a result.
Avanti West Coast and West Midlands Railway are providing alternative journeys for GWR ticketholders.
A GWR spokesperson said: "We have had to cancel a significant number of long-distance train services on Saturday and disruption is expected to continue on Sunday and into the following week.
"Train timetables are presently being worked on and will be published as soon as possible."
LNER services between Edinburgh, Newcastle, York, and London are also affected and the operator has also urged people not to travel on Sunday.
A spokesperson said: "Our sincere apologies to customers affected by disruption. Please do not travel this weekend.
"Tickets dated for travel 8-9 May will be valid up to and including Sunday 16 May."
Hull Trains and TransPennine Express (TPE) were also affected but Hull Trains said its normal service had resumed by Saturday afternoon.
There was no update from TPE for Sunday, although it had said on Saturday that there was a "significant impact" on its Newcastle to York service.
Hitachi Rail said the issue was caused by "cracks on the lifting points under the carriage of some class 800 trains" which were spotted during routine checks.
A spokesperson said: "Safety is our number one priority and as a precaution, the decision was taken to halt the entry into service of our intercity fleets pending inspection.
"We understand the frustration caused and we would like to apologise for the inconvenience caused to passengers and operators.
"Having been cleared for service, some trains are now running again across the network."
Rail minister Chris Heaton-Harris said: "I share the frustration of passengers who are experiencing significant disruption, and would ask people whose journeys are affected to check before travelling."
The non-ministerial government department responsible for regulating Britain’s railways, the Office of Rail and Road, has begun an investigation into the issue.