Workers on the Heathrow Express are to stage more strikes in a dispute over disciplinary action taken against two employees.
Members of the Rail Maritime and Transport union will walk out for nine hours from 0300 GMT on 24 March and for 12 hours from 1100 GMT on 25 March.
The union has held two strikes in protest at the sacking of a driver and action against another worker.
Heathrow Express said the issue was "no longer under dispute".
A spokeswoman for Heathrow Express, which runs services from the airport to Paddington Station, said: "The suspended employee has already returned to work and the dismissal - which was conducted via a policy agreed with the union - has since been withdrawn from employment tribunal."
The RMT's general secretary Bob Crow said: "The unfair and disproportionate punishment taken by management in both these cases is a total miscarriage of justice and that is reflected in overwhelming ballot results for action.
"Management's behaviour clearly demonstrates their willingness to victimise staff and union representatives at the slightest provocation and no doubt they see this as payback for the successful building of a strong and militant union organisation within the company."