Passengers have been warned they will face disruption to services across Yorkshire as members of the Rail, Maritime and Transport union (RMT) stage industrial action for the second time this week.
Long-term arrangements have been negotiated in Scotland and Wales, but talks on the issue have stalled with a number of rail operators in England.
RMT general secretary Mick Cash has previously said its efforts to reach a negotiated settlement with Northern Rail over safe operation and safe staffing had been "kicked back in our faces."
He said the dispute was about putting the safety of the travelling public before the profits of the private train companies.
The company said it will run almost 1,400 services today, with the majority operating between 7am and 7pm.
"The overall number of trains running will be reduced, we expect trains and any replacement buses operate to be extremely busy," it said. "Please allow extra time for journeys, plan carefully and consider whether travel is necessary.
"During the RMT strike action we expect all services to be busy, especially in the morning and evening peak periods, and advise you to allow extra time to travel."