Great Western Railway operator FirstGroup said sales in the first three months of its financial year slipped, adding it was "too soon to judge" how Brexit would affect business.
The transport firm said group first-quarter revenues fell 1.4% as its UK bus operations and US coach service Greyhound weighed on rises at its rail unit and other operations.
But the group, which also runs the TransPennine Express and more than 6,000 buses in 40 UK towns and cities, said a weaker pound would boost sales.
It added that two-thirds of its adjusted operating profit is made in the US, where it also runs school buses.
The firm said: "The weakening of sterling since the referendum outcome would, if maintained, result in translation benefits from our US dollar-denominated businesses, albeit partly mitigated by some US dollar-denominated costs incurred in the UK divisions (principally for fuel), and some US dollar interest and tax costs."
It added that revenues at its UK bus division, which operates in cities such as Manchester, Bristol and Glasgow, fell by 1.4% because lower high street footfall and urban congestion saw passenger numbers decline.
Chief executive Tim O'Toole said: "The group expects to make strong progress in the current year despite a challenging and uncertain trading environment in several of our markets."