Workers on Virgin Trains East Coast are to stage a 48-hour strike in a row over the role of guards and jobs.
Members of the Rail, Maritime and Transport (RMT) union will walk out on April 28 and 29, following strikes at three other train operators last Saturday in bitter rows over staffing.
The RMT said it has been seeking an "explicit clarification" from Virgin on the retention of the role of the guard.
It wants a specific assurance that a new position of train manager will retain the safety- critical roles and training currently held by train guards.
A statement said: "The only response the company has offered is to repeat the vague and non-committal mantra of 'within our discussions we have confirmed that the safety-critical duties of the guard will remain on the train'.
"This mealy-mouthed form of words gives no reassurance to RMT members in the front line nor any protection from the possible introduction of driver-only operation."
The RMT said consultation over "widespread on-board changes" has been going on for more than a year, adding that the company had implemented the changes from March with no agreement with the union.
"The new service is not fit for purpose and has turned into a chaotic shambles, with untrained staff forced into roles and the whole train crew left to work it out for themselves," the union said.
RMT general secretary Mick Cash said: "It is simply appalling that Virgin East Coast have refused to give the most basic assurances on the safety-critical role of the guard on their trains continuing into the future.
"Instead they have sought to fob us off with meaningless statements that are, frankly, an insult to the intelligence of our members who are charged with maintaining safety on board these intercity trains.
"On top of that, chaotic operational changes have been bulldozed through and there is now an additional threat to stations and travel centre staff that leaves our members on a knife-edge.
"This contemptuous attitude has unleashed a tidal wave of anger at the grassroots and the action we have agreed to put on is simply a reflection of the disgust and frustration at months of ducking and diving by the company."
The RMT is embroiled in disputes over staffing and driver-only trains at Southern, Merseyrail and Arriva Trains North.
Its members at the three operators staged a 24-hour strike last Saturday, the day of the Grand National.
The union has taken 31 days of strike action against Southern in the past year, although fresh talks will be held later this month to try to break the deadlock.
Talks about staffing on new Merseyrail trains are also due to go ahead.