Bidders will need relevant railway experience maintaining stations. Tasks include planned and reactive maintenance services across 34 locations on routes from London Euston to Rugby; Bletchley to Bedford and Watford to St. Albans.
The three-year term contract will get underway in September.
Separately, the train operator is also due to start major improvements at Watford Junction and Milton Keynes stations in April.
The £3m package of works will include improvements to station concourses and refurbishment of existing facilities and pedestrian bridge upgrades.