The incident occurred at about 5 am in Prachin Buri, 100 kilometres north-east of Bangkok, Police Sergeant Chaimongkol Chaiya said.
"According to the autopsy, the man had suffered a heart attack and was most likely dead when the train ran over him," Chaimongkol said by phone.
He said the victim identified as Alan George Broen, 68, had fallen on the tracks with his right leg hanging over the rail.
"We are trying to get more details on the man's background," Chaimongkol said.//DPA