Members of the Rail, Maritime and Transport union (RMT) on CrossCountry, walked out on Saturday morning for 48 hours in a squabble over rotas and Sunday working.
Due to the industrial action, no CrossCountry services will operate on both Saturday and Sunday on the Aberdeen to Glasgow line - while reduced services will be in place across other parts of the network.
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.”
Trains have been cancelled, affecting Christmas shoppers on one of the busiest weekends in the lead-up to Christmas.
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.”