Two days of strike action by Arriva Trains Wales drivers in a dispute over pay have begun.It means there will be disruption to services until Tuesday and no alternative transport is being laid on.
The operator said a handful of drivers had turned up to work on Sunday and it was running a sporadic service.
Managers say an offer of a 12% rise over two years is extremely generous, but the Aslef union disagrees.
The union has accused the company of paying Welsh drivers less than their English counterparts.
It called for industrial action on Monday and asked for drivers working overtime on the Sunday shift not to turn up for work in protest.
The company runs services throughout Wales and into parts of England.
The 01.50 GMT Fishguard Harbour to Cardiff service on Sunday was replaced by a bus service.
An Arriva spokesman said four drivers had turned up to work so it had been able to run some services.
He said: "There's a possibility more will turn up during the day but it's a changing situation.
"There are no advertised times for the service because we can't guarantee we'll be able to offer people a return journey."
The operator is running a shuttle service from Pontypridd to Cardiff.
Passengers on all services are being advised to make alternative travel arrangements.
Services on Tuesday are also likely to be disrupted.
Peter Leppard, operations director for Arriva Trains Wales, said: "Arriva Trains Wales' senior management and officials from the Aslef and RMT trade unions have been in talks for many months in an attempt to reach a final agreement on pay and working conditions for train drivers.
"We are extremely disappointed that once again our generous offer has been rejected and that the planned industrial action by Aslef for 27 and 28 February will go ahead."
He said the strike would cause significant inconvenience to customers.
An Aslef spokesman said: "We regret the disruption to the service and our passengers but we would not have staged the strike if there had been any alternatives."
For service updates customers are advised to check the Arriva Trains Wales website - www.arrivatrainswales.co.uk or contact National Rail Enquiries on 08457 48 49 50.