“Fortunately no one has died,” the BTP tweeted shortly after 9:00 pm local time (5:00 pm EDT) on Sunday.
Officers continue to respond to the incident in the Fisherton Tunnel, the agency added.
Earlier Sunday, BTP officers had been called in to help, “following reports that a train derailed.”
A Wiltshire police tweet also said police officers were responding with fire and ambulance service “at the scene of a train accident involving two trains between Andover and Salisbury.”
A statement from Network Rail Wessex said the incident involved the 17:08 pm Great Western Railway service between Portsmouth Harbor and Bristol Temple Meads and the 5:20 pm South Western Railway service from London Waterloo to Honiton.
The line remains closed while emergency services carry out their work, Network Rail Wessex added.