Capt Ranong Chumpinit told the BBC that Daniel Clarke was found at 01:05 GMT on Saturday lying by the train track in Thung-Kha, Chumphon province.
He said Mr Clarke, from Aldershot, told police that he stepped out to smoke between two carriages when he fell.
The Foreign Office said a Briton had been hospitalised in Thailand.
"We are supporting the family of a British national who has been hospitalised in Thailand," a spokeswoman said.
Capt Ranong said a friend of the backpacker told police it was an accident.
He said: "We don't believe there's a foul play going on because his belongings remained intact."

The North East Counter Terrorism Unit confirmed the Dewsbury teenager was questioned in April under section 5 of the Terrorism Act and later bailed.
Hassan Munshi and Talha Asmal, both 17, are feared to have travelled to Syria to join Islamic State in March.
West Yorkshire Police and counter-terrorism officers are investigating.
Speaking at the time of their disappearance, the boys' families said they were praying for the safe return of the "ordinary Yorkshire lads".
Hassan's brother, Hammaad Munshi, was arrested in 2006 at the age of 16 after police found a guide to making napalm on his computer.
He became the youngest person to be convicted under the Terrorism Act.

Lancashire, Manchester and North Merseyside Wildlife Trust said the skins of insects have been found on Winmarleigh Moss, near Garstang.
The baculovirus drives caterpillars on a "death march" to the top of plants, then when it dies the bug bursts out to seek other victims.
The trust's Dr Chris Miller said it was "gruesome - like a zombie horror film".
Dr Miller was carrying out a butterfly survey on Winmarleigh Moss when he noticed a caterpillar hanging from the end of a branch of a small bush.
He then saw another one hanging from a tall blade of grass.
"Both were dead but otherwise intact," he said.
Dr Miller also noticed "small scraps of caterpillar skin" on other branches he checked.
He said research is showing that the baculovirus affects the way the "zombie" insects respond to light, "making them climb to higher and more dangerous places and when they get there they die".
"It is really unusual seeing caterpillars high up as they can be eaten by birds.
"This is a caterpillar of the oak eggar moth which eats heather and bilberry so it is normally hidden in the undergrowth, not at the top of plants."
Source: Lancashire, Manchester and North Merseyside Wildlife Trust
A 21-year-old British man is in hospital in Thailand with head and leg injuries after he fell out of a moving train, Thai police have said.
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A 17-year-old boy has been arrested on suspicion of preparing terrorist acts as part of a search for two teenagers from Dewsbury thought to be in Syria.
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Caterpillars are being killed by a bug which turns them into "exploding zombies", a wildlife expert has said.