The Competition Commission is probing plans by the FirstGroup Plc to run the East Coast mainline rail franchise.
Competition Commission officials are pondering the bid
The service is currently being run by Great North Eastern Railways (GNER), but there are three other bidders for the lucrative franchise.
First also runs Hull Trains, Scotrail and the Transpennine franchises along with a large number of bus services.
First, Danish Railways and a consortium including Stagecoach and Virgin are battling GNER for the contract.
Commission officials say that following an initial inquiry into the application they want to concentrate on any possible overlap between the East Coast rail services First would run and its other operations.
Experts are now considering whether the possible acquisition of the prime service would lead to a lessening of competition in the number of transport services for passengers along the route.
Competition mandarins added that their so-called "issues" statement does not imply any concerns have been identified, but only that interested parties are now being given the opportunity to offer their opinions about the bid.