Four people have been arrested in connection with an ongoing fraud inquiry, understood to relate to rail contracts in London.
Three of the arrests were made on 28 February, and one on 6 March, according to the British Transport Police.
The inquiry is understood to relate to Network rail contracts.
Network Rail told CN none of its employees had been arrested.
All those arrested have been released on bail.
CN understands that no raids have been carried out in connection with the investigation.
The British Transport Police declined to confirm the names or company affiliations of any of those involved.
Network Rail said it could not confirm whether its activities were connected to the police inquiry.