Drivers working for Arriva Trains Wales have voted in favour of strike action in a dispute over pay.
An "overwhelming majority" of Aslef members voted for action, which could start next Friday, over what officials call a "derisory" pay offer.
Arriva Train Wales said negotiations are continuing with unions to reach an agreement which "we hope will avoid the need to disrupt train services".
Action could be taken over three separate days, starting on 19 November.
The Rail Maritime and Transport (RMT) union is also balloting its members.
A meeting between union and company bosses is due to take place on Monday.
Aslef is said to have about 500 members employed by Arriva Trains Wales. The RMT has a smaller number of members.
Union members are reported as being "appalled at the derisory pay offer for 2010" and "fed up with the contemptuous attitude of the company".
Aslef Wales district organiser Stan Moran said drivers wanted a "fair deal", claiming they earned considerably less than colleagues employed by other train companies.
The RMT says their pay negotiations with Arriva Trains Wales had ground to a halt, leaving it with "no option" but to begin balloting for action.
General secretary Bob Crow said: "RMT have totally rejected a contemptuous pay 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.
"RMT is making it clear that this stalemate on pay at Arriva Trains Wales can no longer be tolerated."
Aslef organiser Mr Moran said 24 hour strike dates would be given to the train operator - 19, 26 and 27 November.
Arriva Train Wales said they had yet to receive confirmation of the strike dates.