First Great Western workers have agreed to stage a series of strikes including over the August bank holiday weekend.
Members of the Rail, Maritime and Transport union, including drivers and guards, will walk out for 24 hours on August 23 and for 72 hours from August 29, with other members who work on maintenance striking on August 29 and 31.
They are in dispute over the introduction of new Hitachi intercity trains.
Mick Cash, the RMT general secretary, said: "Despite strenuous efforts by our negotiators we have not been able to secure the kind of progress required in the key areas of jobs, services and safety for us to reach an agreement. As a result we have no option but to put further action on."
First Great Western runs trains to and from London Paddington, south Wales and the south-west.