Members of the Rail, Maritime and Transport (RMT) union on CrossCountry walked out this morning, at the beginning of one of the year's busiest shopping weekends.
CrossCountry operates a number of services across the region.
RMT general secretary Mick Cash said: "It is disgraceful that the management effectively sabotaged talks this week that were making progress towards a settlement to this dispute.
"It is the company's ineptitude and arrogance that has forced us to take a further round of action this weekend.
"Our members are standing up for the principles of a decent work-life balance and against the abuse of rostering and Sunday working by a management that is out of control.
"The action is rock solid across the franchise from Penzance to Aberdeen today and the disruption to services is solely down to management's intransigence."
A spokesman for CrossCountry said: "It is disappointing that the RMT union continues to instruct its members to strike this weekend, and has now added additional dates over Christmas that will further inconvenience people wanting to travel. We have again asked the RMT to call off this unnecessary and unwarranted action, and to consult their members on the offer we have made.
"We will operate a limited service on both dates, and other train operators and many Arriva buses have agreed to accept CrossCountry tickets.
"Details of the revised timetables, along with alternative travel arrangements and options for using tickets that have already been purchased, are available on our website."