Simon Brown, 24, from East Grinstead, West Sussex, hit his head on a signal gantry on 7 August.
The hearing in London was told that passenger Kirstin Duffield heard a "loud thud".
In a statement read by the coroner, she said she found Mr Brown with an injury that was "not survivable".
The train from Gatwick to London Victoria was travelling at about 60mph when the incident happened near Balham, south London.
Mark Young, from the Rail Accident Investigation Branch (RAIB), told the hearing Mr Brown's head was out of the window with no evidence to indicate why.
He said Mr Brown suffered a fatal head injury as a result of striking a signal gantry.
Ms Duffield said she saw Mr Brown had collapsed in the corridor after she stood up from her seat, but saw he was still breathing.
"There was a lot of blood on the floor and around his head. There was a massive trauma to the top of his head," she said.
She said it became apparent there was nothing she could do for him.
The inquest heard Ms Duffield got off the train at Wandsworth Common after the alarm was raised and the driver had been alerted.
She said she saw "blood splatter" on the outside of the carriage, but had not seen Mr Brown with his head out of the window.
Mr Brown's mother, Jane Street, said her son had a passion for railways and "was neither reckless nor ignorant of the dangers of that environment".
In a statement, she said her son first volunteered on the Bluebell Railway as a nine-year-old and had recently become an engineering technician with Hitachi Rail Europe in Bristol.
Mr Brown's father, Mike Brown, said his son had been due to move in with his girlfriend and he had "never seen him so relaxed, happy and enthusiastic about his future".
Questioned about findings by the RAIB, Mr Young said the distance between the window and gantry was found to be 26cm while the train was static.
He said it complied with standards for existing structures but was less than an industry-recommended minimum for new structures.
He also said it had been found the distance between the gantry and a moving carriage could have been as little as 68mm.
He said the window opposite the guard's compartment was not intended for passenger use, although it was accessible to anyone on the train and open when the train left Gatwick.
A yellow sticker on the door warning people not to lean out of the window was "in a rather cluttered environment" among many other signs, he noted.
The inquest continues.
A man who died on the Gatwick Express was found by another passenger with a "massive trauma" to his head, an inquest has heard.