Frozen compacted leaves were the cause of a near-disaster in Surbiton last November when a packed commuter train shot through two red lights, according to a long-awaited technical report.

But it remains unclear why conditions on this stretch of line were severe enough to cause this rare type of problem, and Network Rail has promised to monitor it extremely closely this autumn.

Driver and mechanical error were ruled out soon after the near-miss on November 25, which involved the 5.44am South West Trains service from Alton to Waterloo. The train went through two red lights and stopped just 300 yards from the back of another train, despite the driver slamming on the brakes.

Network Rail was put in charge of an investigation, which involved extensive testing at a Siemens state-of-the-art testing track at Wildenrath in Germany, to recreate the conditions of the accident.

Stuart Buss, a spokesman for Network Rail, said the resulting report which was published two weeks ago would not be made fully public and was highly technical.

He said the signals had operated in time and as normal and that the train had skidded through both red lights after the driver applied the brakes. The cause of the near-miss seemed to be bad track conditions.

He said: "Each autumn thousands of tonnes of leaves fall on the railway line, which when compressed by passing trains, create a thin, black Teflon-like layer on the rail. This compromises train braking and acceleration.

"Network Rail and our industry partners invest millions of pounds yearly to tackle these problems including using multi-purpose vehicles fitted with lasers to blast leaf mulch off the track, an extensive vegetation management programme, dedicated leaf-busting teams, and extensive testing of trains in autumn conditions.

"In addition to this and in direct response to the report, Network Rail will work closely with the train operating companies on a number of actions to overcome this issue before the start of next autumn."

Mr Buss said the company will watch this stretch of track carefully throughout autumn, using extensive treatments against leaf problems.

But Tony Adams from Kingston Area Travellers' Association was sceptical. He called for the report to be made fully public, saying: "I think they should show us the report. What are they hiding?

"We are talking about a public rail service, not corporate research. They know we get autumn every year, so why weren't the tracks treated carefully?"