The Competition Commission (CC) is to launch an inquiry into whether the acquisition of the West rail franchise by FirstGroup threatens fair trade.
FirstGroup operates rail and freight train services across the UK
The move follows the Department of Transport's announcement on 13 December that FirstGroup had won the new franchise from April 2006.
Of particular concern was the supply of passenger services on shorter journeys.
A spokesman for the CC said pricing on those journeys may constitute unfair competition to bus routes.
The spokesman added: "The CC is required to publish its final report on the acquisition by FirstGroup by March 16 but intends to publish its provisional findings report by the end of January."
FirstGroup's West franchises are worth more than £1bn in revenues a year.
The firm is paying the government £1.81bn for the two franchises, which both begin on 1 April 2006.