Three more days of strikes are now planned in addition to the three walkouts already announced, starting this weekend.
The news means Saturday services on Northern Rail will be affected for the next six weeks as the RMT intensifies its action over the introduction of driver-only trains.
The union claims the additional strikes are in response to the company refusing to enter into “serious talks.”
Rail passengers will now face problems on Northern services this Saturday, followed by September 1, 8, 15, 22 and 29.
RMT General Secretary Mick Cash said: “RMT has made every effort to get serious and meaningful talks going with Northern.
“But instead of making progress towards a settlement that matches the best practice in the rail industry as negotiated elsewhere the company have opted instead to play and fast and loose making a mockery of the exercise.
“It is the pig-headed and arrogant attitude of the company that has forced us to announce this next phase of action and to confirm that the strikes planned from Saturday go ahead.
“RMT members on Arriva Rail North have been in dispute for well over a year now in a battle to put public safety before private profit and we are angry and frustrated that a genuine opportunity to reach an agreement has been kicked back by the company.
“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 and they should listen to their front-line staff and pull back from that plan immediately.
“RMT recently secured an agreement on Greater Anglia that enshrine the guard guarantee. Similar agreements have also been reached in Wales and Scotland. Arriva Rail North need to do the right thing and come to an agreement that secures a guard on their trains too.
“We thank the public for their support and understanding throughout this dispute over rail safety and access and the union remains ready for genuine and serious talks.”