“Luckily no one is dead,” BTP tweeted shortly after 9pm local time (5pm EDT) on Sunday.
Officers continue to respond to the incident at the Fisherton Tunnel, the agency added.
Earlier Sunday, BTP officers had been called in to help, “after reports of a train derailed.”
A tweet from Wiltshire Police also said police officers responded 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 17:08 Great Western Railway service between Portsmouth Harbor and Bristol Temple Meads and 17:20 South Western Railway from London Waterloo to Honiton.
The line remains closed while emergency services perform their work, Network Rail Wessex added.