Passengers are believed to have been injured after a crash involving two trains in Salisbury.
The incident happened when a train carriage was derailed after hitting an object, and a second train then collided with it when signalling was affected.
There were unconfirmed reports that a train driver is trapped at the scene.
Sources told the PA news agency that up to a dozen passengers may have been injured, however it is understood that nobody died in the incident.
All passengers have been evacuated and injuries are believed to be minor, with the sources describing people as "walking wounded".
Emergency services, including some 50 firefighters are at the scene close to London Road, Salisbury.
A Network Rail spokesperson said: "At around 7pm this evening, the rear carriage of the 1708 Great Western Railway service from Portsmouth Harbour to Bristol Temple Meads derailed after striking an object on its approach to Salisbury station.
"The derailment knocked out all of the signalling in the area. Subsequently, the 1720 South Western Railway service from London Waterloo to Honiton then collided with the Bristol train.
"There are reports of injuries and the emergency services are on site along with railway first responders."
The derailed train is believed to be on its side, but the South Western Railway train is upright.
A critical incident has been declared and the line will remain closed at the country end of Salisbury.
British Transport Police said a casualty centre has been set up at St Mark's Church, in St Mark's Avenue in the city.
A spokesman tweeted: "Officers are continuing to respond to the incident at Fisherton Tunnel. A number of people have been injured, but thankful no-one has died."
