Northern Rail workers have announced a fresh series of 24-hour strikes, with the first planned for 25 August.
Staff will also walk out for 24 hours on 1 September and 8 September, the RMT Union announced.
The union is in a dispute over the provider's plan to change provision of guards on trains, which they say compromises passenger safety.
“German-owned Northern Rail want to run nearly half a million trains a year without a safety critical guard on board in a move that would wreck both safety and access to services," RMT general secretary Mick Cash said in a statement.
"They should listen to their front-line staff and pull back from that plan immediately.”
The union has also accused Northern Rail of failing to negotiate in good faith.
Northern Rail insists it wants to continue "constructive discussions" with the union, and wants to talk about guaranteeing jobs and pay for its 1,300 conductors for the next eight years.
"Northern is doing everything possible to find a solution to the RMT's dispute and maintains that talking costs nothing, whilst strike action causes inconvenience to customers and damages the case for long-term investment in rail," Richard Allan Northern's deputy managing director, said.
At the same time, another walk-out over the removal of guards, planned for Saturday on South Western Railway, was called off to allow further negotiations.