Daniel Clarke, 20 from Aldershot, had been travelling from Bangkok to Nakhon Si Thammarat in the south of the country.
Police authorities said Clarke was found at 01:05 GMT on Saturday (April 1) lying by the train track in Thung-Kha, Chumphon province, the Telegraph reports.
It was later discovered that three days earlier Clarke had posted a selfie of himself to social media hanging out of a train carriage window.
He was found unconscious by the tracks wearing the same blue vest he was wearing in the picture.
It is not known how long Clarke had been on the train and where he boarded it before falling off.
He was not notice by the driver of the train who told local police and emergency services that he believed Daniel had fallen from the train in front.
The friend pictured in the picture with Clarke, Ross Wild, confirmed that he had now found him but declined to comment further.
Thai police told BBC News they believed Clarke had stepped out to smoke between two carriages when he fell.
A Foreign Office spokesperson in London said: "We are supporting the family of a British national who has been hospitalised in Thailand."
Doctors treating Clarke at the Chumphon Khet Udomsakdi Hospital and said surgeons had operated on him and scans showed he had not suffered brain damage.
