Accident investigators found "no evidence" to explain why a passenger put his head out of a train window, which led to his death by colliding with a signal gantry.

Simon Brown, 24, was travelling on a Gatwick Airport to London Victoria train when the accident occurred near Balham, south London.

The Gatwick Express train was travelling at around 61mph at the time of the incident on August 7 last year, the Rail Accident Investigation Branch (RAIB) said.

A post-mortem report found no traces of drugs or alcohol, and the evidence suggests Mr Brown was not taking photographs, according to the report.

The window was part of a door opposite a guard's compartment and was not intended for passenger use, although it was accessible to anyone travelling on the train.

Investigators noted that a yellow sticker on the door warning people not to lean out of the window when the train is moving was in a "cluttered environment" among other information signs.

"It is not possible to say whether the lack of conspicuity of the warning notice was a factor in the accident," the RAIB said.

The distance between the section of the window where Mr Brown's head was positioned and the signal gantry - believed to have been installed in 1952 - was found to be 26cm.

Although the clearance complied with standards for existing structures, it was less than an industry-recommended minimum for new structures where there are trains with opening windows.

The report recommended that Network Rail - in collaboration with train operators - introduces a process for sharing data regarding clearances between structures and trains at window height, so operators can make "more informed decisions" about the management of risk associated with opening windows.

Mr Brown was pronounced dead at the scene by ambulance staff. British Transport Police treated the incident as non-suspicious.

He was described by friends as a "life-long railway fanatic".

Originally from East Grinstead, he first volunteered on the Bluebell Railway as a nine-year-old and had recently taken up a position as an engineering technician with Hitachi Rail Europe in Bristol.