A lone boatman watched the spectacle of Thailand's so-called "floating train" as selfie-seeking passengers soaked up the water views after disembarking from carriages stopped on a narrow bridge.

Railway adventures are gaining popularity in the kingdom with tourists seeking off-the-beaten-track travel experiences away from the hordes visiting temples and beaches.

Saturday was the State Railway of Thailand's first Bangkok to Pasak Jolasid Dam service "Rot Fai Loi Nam" marking the end of the rainy season.

Hundreds of bleary-eyed passengers boarded the train at Bangkok's century-old Hua Lamphong Station before sunrise.

Lotus ponds, jungles, temples and rice paddies zipped by as tourists in third class hung out the window and took pictures while enjoying the natural air conditioning.

Greying Thai aunties boarded at Ayutthaya, the ancient former capital of Siam — to sell fairy floss and pad krapow, a famous stir-fry basil dish.

Three and a half hours after chugging out of Bangkok, the refurbished Japanese train pulling more than a dozen carriages crossed the Pasak Jolasid reservoir over a series of viaducts and stopped for 20 minutes of selfie time.

For most of the year, cattle graze underneath the bridge "but from October to January, the water is high on both sides which gives the impression that the train is floating on water," said long-time British expat, train enthusiast and travel blogger Richard Barrow.

"I have done it many times and I am already booked to do it three more times this season."

Barrow lamented the trip was only publicised in Thai language and not English so not many foreigners know about the unique experience.

"It's kind of a missed opportunity," he said.

Enjoying the day trip with her mother, 11-year-old Lily Piratchakit said the water view was "endless".

"It was amazing," she told AFP.