Rail firm GNER is going to court to try to overturn a decision by the Office of Rail Regulation (ORR) allowing a rival firm to operate on its routes.
The ORR says it will "vigorously contest" the proceedings by GNER
GNER, which runs intercity trains on the East Coast Main Line, argues that ORR's decision to allow Grand Central to operate on the route is unlawful.
GNER says it pays more to access the track than Grand Central, which is set to run a London to Sunderland service.
A spokesman for the ORR said it would "vigorously contest the proceedings".
GNER, which has a franchise agreement to run intercity trains on the East Coast Main Line, is focusing on the rates it pays in track access charges compared with Grand Central, which is an open access operator.
As a franchise rail operator, GNER has to make three payments to gain access to the tracks: a fixed charge, a variable charge, and a franchise premium of £1.3bn over 10 years.
In March, Grand Central won the right to run three direct trains a day between Sunderland and London.
GNER believes the ORR decision is "unlawful because Grand Central would not pay either the fixed track access charges or premium payments that GNER pays under its franchise contract".
In addition GNER is challenging Grand Central's plan to stop at York, "which is already served by 61 trains a day to and from London, when its stated goal is to create a new rail market between Sunderland and London".
Another grievance of GNER is that it would lose money to Grand Central since it would have to share revenue with the firm.
GNER stressed that the case is against the ORR not against Grand Central.
The court case, which is taking place in London, starts on Tuesday and is set to run until Friday.