The driver is among those who have been hospitalised following the incident on Sunday evening.
British Transport Police (BTP) said later that night no one had died in the crash but “a number” had been injured.
The incident happened after the rear carriage of one train derailed in the Fisherton Tunnel in Salisbury. Another train then crashed into it after signalling was damaged.
After the collision, the derailed train - a Great Western Railway (GRW) service from Portsmouth Harbour to Bristol Temple Meads - was believed to be on its side.
The other involved - a South Western Railway (SWR) train from London Waterloo to Honiton - remained upright.
BTP said they were called to Fisherton Tunnel in the southern English city at 6.46pm on Sunday “following reports of a train derailment which involved two passenger trains colliding”.
The force said: “Thankfully there have been no fatalities however a number of people have been injured and a casualty centre has been opened at a nearby church.
“Most of these people are walking wounded however a small number, including the driver, have been taken to hospital where their injuries are being assessed.”
Firefighters said around 100 people were evacuated from the scene in the tunnel, which is on the eastern side of Salisbury.
The driver was trapped in his cab and had to be cut free following the crash.
Martin Frobisher, group safety and engineering director, technical authority, at Network Rail, said on Monday morning he did not know exactly what happened in the crash the night before.
“We’re hugely relieved that nobody was seriously injured, but the passengers must have had a really scary experience, and we’re very sorry for that,” he told BBC Radio 4’s Today programme.
“We’re obviously starting now a very detailed and forensic investigation into what happened.”
BTP said on Sunday night the line was expected to remain shut “for some time”.
GWR said the railway in the area would remain closed on Monday as investigations continued. “specialist teams continue their investigations”.
It urged customers not to travel on the affected part of the network, adding the closure would affect SWR trains between Exeter and Basingstoke and GWR trains between Westbury and Portsmouth.
Additional reporting by Press Association
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