Babcock Rail said it expected the job losses would be split between Scotland and the north of England and would include a "slimmed down" management team.
Chief executive John Howie said: "Our business transformation plan is designed to make the business fit for the future.
"We very much regret that jobs will be lost, but our new structure will make the rail division a stronger organisation, providing greater stability for the future and for the majority of our people, who will stay with the business."
Bob Crow, general secretary of the Rail Maritime and Transport union, said: "The axing of 300 jobs by Babcock Rail is another kick in the teeth for the rail industry in the UK and comes on top of plans by Network Rail to sack 1,500 maintenance staff.
"Essential renewals and repairs work is being scrapped to save money with skilled workers dumped on the dole. The jobs massacre on our rail tracks will have lethal consequences for the travelling public."
The move comes after Mr Crow announced plans to ballot for industrial action among Network Rail staff over pay and conditions.