Three new strikes are planned by Northern rail workers in their dispute about the role of guards, including one over the August Bank Holiday weekend.
Rail, Maritime and Transport union (RMT) members plan to walk out on 25 August, and 1 and 8 September.
The union says 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".
Mick Cash, RMT general secretary, claimed Northern management had made a "mockery" of talks and "strung along" union reps.
The dispute has lasted 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 17 August.