Deutsche Bahn (DB) staffers are being investigated over corruption allegations following accusations that the company used bribes to win contracts, according to reports.
DB, which started its own investigation at the end of 2009, said the money involved in the past five years ran into a few million euros.
A Deutsche Bahn spokesman said the investigation was in no way connected to the ongoing takeover of UK transport operator Arriva.
German prosecutors are expected to press charges against DB staffers.
The allegations come a day after the German-based group made a bid of €1.85bn ($2.5bn) for Arriva.