Virgin Trains' legal challenge is an application for a judicial review of the decision to award FirstGroup, the West Coast main line contract.
The firm lodged papers at the High Court in London today and a hearing will be held at the court in due course.
It is unclear if this means the final signing of the franchise contract by the Transport Secretary Justine Greening - which is expected tomorrow - will now be delayed pending the outcome of the hearing.
Remember the Virgin Trains mess? Later an official report into what went wrong will be made public.
The Government has been forced to admit the profitable West Coast Main Line contract has yet to be signed, thanks to a legal bid by Virgin.