A 24-hour walk-out of train staff on Valley Lines services will be held on Monday.
Staff at Arriva Trains Wales represented by the RMT union are in a dispute over pay and conditions.
RMT General Secretary Bob Crow said: “RMT has rejected a paltry offer from the company that would have left our members out of pocket and which was loaded with strings. Our members would have ended up with inferior working conditions for a de facto pay cut.
“Arriva Trains Wales have shown us in the clearest possible terms that they have no wish to properly reward the hard work and loyalty of staff and are motivated solely by the drive for bigger profits to line the pockets of the shareholders. That is why we have had no choice but to strike on December 27.
“With the action now on, it is down to Arriva Trains Wales management to get back to the table to seriously negotiate a pay offer which improves the work/life balance of drivers and which properly rewards their staff.”