Three new strikes are planned by Northern Rail, including one over the August Bank Holiday weekend.
NR workers are continuing their dispute about the role of guards.
Rail, Maritime and Transport union (RMT) members are now set to walk out on Saturday, August 25, and September 1 and 8.
The union insists passenger safety will be put at risk by getting rid of guards and extending driver-only services.
Richard Allan, deputy managing director of Northern, called the new strike dates "disappointing and frustrating".
Mr Allan said the strikes came despite Northern having proposed and entered into a joint working party with RMT, and described the strikes as a "blow to our customers".
However, RMT general secretary Mick Cash has claimed Northern management had made a "mockery" of talks.
The dispute has now been rumbling on for more than a year.
Mr Cash called on bosses at Arriva Rail North - the train operator's parent company - to reach a similar deal to those struck with Greater Anglia, and in Scotland and Wales.
Northern has proposed further talks for next Friday, August 17.
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