Northern rail passengers have been warned to expect "extremely busy" services, as staff stage a 24-hour strike.
Members of the RMT union walked out on Saturday in a long-running dispute over the role of guards.
RMT general secretary Mick Cash said members were striking to "protect passenger safety".
Franchise operator Arriva Rail North said most trains will run between 07:00 and 17:00 BST.
The row has been going on for more than a year, with other rail franchises involved in similar disputes.
The firm faced widespread criticism over cancellations, delays and the implementation of a new timetable on Monday.
It was announced on Friday that there would be an independent investigation in to the recent disruption to services.
Mr Cash said RMT members were "standing firm again".
He said: "RMT is clear, the only way that you can guarantee disabled access and passenger safety is to guarantee a guard on the train."
The company said for passengers should allow "extra time for journeys" and "consider whether travel is necessary".
"As the overall number of trains running will be reduced, we expect trains and any replacement buses we operate to be extremely busy," Arriva Rail North said.