Cleaners on First Great Western trains at Cardiff and Swansea depots are to strike for 24 hours from 6am on Friday.
The RMT union members, who work for the Mitie company, voted by 100 percent to strike over pensions, payment of wages, communication failures and the failure to make a pay offer last year.
Mitie recently announced a 12 increase in its profits.
Bob Crow, the RMT general secretary, said, “This appalling situation shines the spotlight on just how these facilities companies operate and how they treat their low‑paid staff.
“As a result of these combined attacks on working conditions RMT members have returned a rock-solid mandate for action and will strike.”