A number of people were injured and a driver was trapped in his cab when two trains collided in Salisbury.

The incident happened when a carriage was derailed after hitting an object, and a second train then crashed into it when signalling was damaged.

British Transport Police (BTP) said “a number of people” were injured in the incident, at Fisherton Tunnel close to London Road in the city.

The fire service, who sent some 50 firefighters to the scene on Sunday evening with a critical incident being declared, said it had helped to evacuate approximately 100 people.

In a statement late on Sunday night, British Transport Police said of those injured: “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.”

The driver had been trapped after the crash but was released from his cab having sustained injuries that were not believed to be serious, sources confirmed to the PA news agency.

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.”

After the incident unfolded, the derailed train was believed to be on its side, but the South Western Railway train was upright.

BTP said a casualty centre has been set up at St Mark’s Church, in St Mark’s Avenue in the city and said officers would remain on scene throughout the night as they work to “establish the full circumstances of how this incident came to happen”.

They added that line is expected to be shut “for some time”.

Transport Secretary Grant Shapps tweeted: “My thoughts go out to those affected by the serious rail incident near Salisbury.”

He said the Rail Accident Investigation Branch and Office of Rail and Road will investigate, adding: “We need to understand how this happened to prevent in the future.”

A mother who was out trick or treating with her family nearby likened the noise of the crash to “a bomb” and “thunder”.

Tamar Vellacott said she was walking outside with her young children, mother and partner on Jewell Close, Bishopdown, around a kilometre from the scene.

“It was a noise we’ve never heard before… my young ones started panicking thinking it was a bomb and we said maybe a lorry had crashed on the London Road and not to panic,” the 25-year-old told PA.

“There was no screeching like brakes, just a long rumbling sound like thunder hitting the railway line.”

An Office of Rail and Road spokesperson said: “We’re supporting Network Rail and the train operators, plus RAIB (Rail Accident Investigation Branch) and the British Transport Police with respect to the collision between two trains near Salisbury Tunnel Junction and liaising with emergency services responding to the incident.”

Transport Salaried Staffs Association general secretary Manuel Cortes said the incident was “a very sobering reminder about why safety on our railways is always paramount”.