Workers on Virgin Trains West Coast are said to be “solidly supporting” a strike over pay.
Members of the Rail, Maritime and Transport union and the Transport Salaried Staffs Association mounted picket lines outside stations at the start of the 24-hour walkout.
Virgin said most of its services would run despite the action.
RMT general secretary Mick Cash said: “Our members the length and breadth of the Virgin West Coast operation are standing solid and determined this morning in their fight for workplace equality and justice.
“Virgin is a wealthy outfit and there is no excuse whatsoever for Sir Richard Branson to undervalue and discriminate against the very same staff whose hard work finances his luxury lifestyle.
“Virgin should stop trying to play divide and rule and should address the fundamental issue of workplace equality which is at the heart of this dispute.”
Virgin said it had offered a “significantly above-inflation pay rise” which had been rejected by the unions.
Phil Whittingham, managing director for Virgin Trains on the west coast, said: “The RMT leadership is attempting to cause disruption when many will want to travel by train to spend time with loved ones.
“We have explored a generous 3.6% pay increase, however the unions’ leaderships are insisting on 4%, double the 2% average increase seen across the UK this year.
“We know how important it is for friends and families to get together over the festive season, so whilst we’re sorry for the disruption we will keep the majority of our trains running with fully-trained staff on board and at stations.
“We remain open to talks with the RMT and TSSA, and urge them to call off these strikes which will cost their members pay for no gain.”
Passengers were urged to check before setting off and warned that services would be busier than usual.