The dispute is a familiar one: the role of guards.
The RMT’s general secretary, Mick Lynch, said: “There has been a systematic attempt by CrossCountry to undermine the role of the senior conductors and train managers by drafting in other staff to do their jobs.
“CrossCountry need to understand that they already have trained and competent guards that are available and ready to work.
“They need to stop with their underhand measures and attempts to train up various other employees to undertake the role of the guard. This strike could be resolved if people were simply allowed to just get on with their own job and the company stopped threatening the long-term futures of our members and compromising safety as a consequence.
“Our members have made it clear that they will not put up with this attack and that is why they are taking action over Christmas and New Year.”
CrossCountry said: “We will only be able to run a very limited timetable, with the cancellation of many of our services.
“We are advising customers to alter their plans where possible and to avoid travel on either Friday 24 December or Friday 31 December.
“If you travel on Friday, then you may use your ticket on any train. Seat reservations will not apply on Friday – please take any available seat in the class shown on your ticket.
“Your ticket may also be accepted with other operators on Friday.”
CrossCountry intends to run a curtailed service between Edinburgh and Plymouth, Manchester Piccadilly and Bournemouth and Birmingham New Street and Leicester.
Links to Aberdeen, Glasgow Central, Nottingham, Cardiff, Paignton and Stansted airport are covered by other operators – apart from between Leicester and Peterborough, on which a bus replacement will run.
Tickets bought for Christmas Eve will be valid on Wednesday 22 or Thursday 23 December, with no need to change them.
A similar pattern is expected to apply in the lead-up to 31 December.
Talks are understood to be continuing between the two sides to try to avert the stoppages.