Salisbury 2021: an SWR train slid through a red signal on leaf slime into a GWR train

At 18:43 on 31 October 2021, the 17:20 South Western Railway service from Waterloo to Honiton passed a red signal at Salisbury Tunnel Junction and ran into the side of the 17:08 Great Western Railway service from Portsmouth to Bristol at 52 mph. The front two carriages of one train and the rear two of the other derailed, and both ploughed into Fisherton Tunnel. Thirteen passengers and one railway worker — the SWR driver, who had applied the brakes — were hospitalised, the driver with life-changing injuries; a head-on with an earlier train was missed by less than a minute.

RAIB's reconstruction is a chain of small failures around one physical fact. Leaf contamination on the railhead, plus a band of drizzle just before the train passed, had cut wheel-rail adhesion to a fraction of normal. The driver braked too late for that level of grip, the train's wheel-slide protection could only release brake pressure to keep the wheels turning — saving steering but not shortening the stop — and the train drifted through the signal with the Train Protection and Warning System firing too late to matter.

Why was the railhead that slippery? RAIB found vegetation density along the approach had increased and had not been effectively managed by Network Rail's Wessex route, and that leaf contamination was controlled that evening by neither proactive nor reactive measures. The probable underlying factor was that the Wessex route did not effectively manage leaf-fall season risks; a possible factor was that South Western Railway had not prepared its drivers to assess and report low adhesion, so the day's grip information died with the crew that felt it.

The report also recorded safety observations: the revised design criteria for TPWS, and how signal-overrun risk assessment accounts for sites with high low-adhesion risk. Two issues worsened the consequences — loss of survival space in the driver's cab and jammed internal sliding doors that obstructed evacuation. No one died; the industry's oldest joke had simply been left unowned at one junction on one autumn evening.

14 of the 92 people aboard hospitalised, the driver's injuries life-changing; two trains derailed, track and junction rebuilt

Vegetation density beside the junction approach had grown without effective control by the infrastructure manager, feeding more leaf mulch onto the railhead.

Neither proactive treatment nor reactive response to railhead contamination was in place that evening, despite the leaf-fall season being a scheduled, known event.

The operator had not prepared drivers to assess and report low adhesion, so a critical real-time signal about track conditions died with the crew that felt it.

The braking system could not mitigate very low adhesion, and the protection systems were positioned as a last line — with nothing managing the hazard they were backing up.

'Leaves on the line' is a punchline until it owns a junction: seasonal risk gets managed only if someone treats the railhead on time and crews report slime before braking distances triple.

RAIB published its final report on 24 October 2023. Network Rail said it was reviewing its training and competence framework for off-track staff and its adhesion management standards, while the Wessex route reviewed how it responds to low-adhesion reports, including railhead treatment timing. The safety observations pushed TPWS design criteria and signal-overrun risk assessment toward high low-adhesion sites. Years of mocked 'leaves on the line' excuses had made the seasonal programme a running joke; Salisbury put it on record as a life-safety system.