Members of the Rail, Maritime and Transport union (RMT) are staging industrial action today and again on Thursday, meaning passengers will face disruption to services across Yorkshire.
Arriva Rail North, which runs Northern, has been embroiled in a year-long argument with the union over staffing and guards.
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 said: “Every single effort that RMT has made to reach a negotiated settlement with Northern Rail over safe operation and safe staffing has been kicked back in our faces.
"No one should be in any doubt, this dispute is about putting the safety of the travelling public before the profits of the private train companies.”
Arriva says it wants to hold constructive discussions with the union about introducing driver controlled trains on parts of its network.
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."