Aslef will ballot its members on Hull Trains on possible strike action.
A fresh dispute is flaring on the railways, threatening disruption to more services in the coming weeks.
The first of a series of weekend strikes by train managers on busy rail routes started on Sunday, causing timetables to be reduced.
Members of the Rail, Maritime and Transport union (RMT) at Avanti West Coast will stage a number of Sunday walkouts in a dispute over rest day working.
Avanti urged its customers to travel either side of Sunday which marked the start of strike action every Sunday until May 25.
Meanwhile the train drivers union Aslef announced it is to ballot its members on Hull Trains for strikes over the sacking of a union member.
Voting will start on January 20 with the result due on February 12.
Aslef said the driver had been unfairly sacked but the company insisted it had followed correct procedures following an investigation.
RMT members mounted picket lines outside stations served by Avanti on Sunday.
A reduced timetable was be in place with fewer services running during limited operating hours.
Kathryn O’Brien, executive director of customer experience at Avanti West Coast, said: “We’re disappointed by the RMT calling strike action for an extended period when our customers may be working, visiting family and friends, or enjoying days out.
“As a result, they will face significantly disrupted journeys during this time.
“I would like to thank them for their patience and understanding.
“On the strike days we’ll have a reduced service, so customers with tickets for those days are strongly advised to travel on alternative dates or claim a full fee-free refund.
“We remain open to working with the RMT to resolve the dispute.”
RMT general secretary Mick Lynch said: “It is wrong that Avanti is paying replacement managers up to £500 per shift—around double what our members earn—while these managers fail to deliver the same service for passengers.
“This destructive approach is typical of train operating companies and stems from the failed policies of the previous Conservative government, which rewarded excessive payouts for managers rather than resolving disputes fairly.“
