On 31 October 2021, Great Western Railway's (GWR) 17:08 Portsmouth Harbour to Bristol Temple Meads four-carriage service (reporting number 1F30) operated by a pair of two-carriage Class 158s - 158762 and 158763, was passing through Salisbury Tunnel Junction where the Wessex Main Line converges with the West of England Main Line.

At the same time, the South Western Railway's (SWR) 17:20 London Waterloo to Honiton service (reporting number 1L53) operated by a single three-carriage Class 159 - 159102, was approaching the junction.

[1][2] The SWR train failed to stop at signal SY31, 240 yards (220 m) from Salisbury Tunnel Junction, which was displaying a red aspect.

National Police Air Service and HM Coastguard despatched helicopters to the scene.

The accident was the first involving a collision between two moving in-service passenger trains in the United Kingdom since the Ladbroke Grove rail crash on 5 October 1999.

[10] The driver of the SWR train was initially trapped in his cab before being freed and taken to hospital, where his injuries were described as life-changing.

[citation needed] The A30 London Road was closed until 9 November to allow for the recovery of the carriages and for investigators to carry out their work.

[18] Rail magazine editor Nigel Harris pointed out that the only facts in the story were the location and services involved.

Under the current system, train operating companies, the British Transport Police, and the Office of Rail and Road can all issue statements.

In his opinion, the initial errors were made worse by Network Rail's Safety and Engineering Director Martin Frobisher, who appeared on BBC Radio 4's Today programme on 1 November and did not correct any of the errors at a time when, according to Wolmar, he must have known them to be untrue.

[22] On 2 November the RAIB stated that initial evidence indicated that, although the SWR driver had applied the brakes, his train had failed to stop at a signal and collided with the GWR train; wheel slide, as a result of low railhead adhesion, was the most likely cause.

[4] RAIB said they would examine how Network Rail managed the risk of loss of adhesion at the track site and also any SWR policy for preventing or mitigating wheel slip on their trains.

SWR commented that its driver had acted in an "impeccable way in a valiant attempt to keep passengers safe".

About one mile (1.5 kilometres) past the signal the train was travelling at 86 mph (138 km/h) and a brake application was made and the wheels began to slide.

[5] Subsequent analysis of the railway line approaching Salisbury Tunnel Junction revealed that the line was contaminated by the residue of fallen leaves between 80 miles 32 chains (129.39 km) and 82 miles 36 chains (132.69 km) (distances measured from London Waterloo).