The train, travelling from Thon Buri Station in Bangkok to Chumphon’s Langsuan Station, derailed as a result of the impact.
The truck, which was carrying cattle, breached a barrier and hit the train in Phetchaburi province.
"Five carriages were derailed... so far I have received reports that three people were killed and between 20 to 30 passengers have been hospitalised," Thanongsak Kongprasert, deputy governor of the State Railway of Thailand,said
The truck driver was among the dead, and two train passengers, including a monk, were also killed, he said, adding that all train services to the South were suspended due to damaged tracks.
Police said it seemed that the truck had a brake failure, resulting in it crashing through the barrier into the passing train. Rescue workers said dead cows were strewn across the track.