Train No. 451 was en route from Nakhon Si Thammarat to Sungai-Kolok border station when its fourth carriage jumped the track at kilometre marker 1140 at Ban To Lua Bae in tambon Sungai-Kolok at 3pm.
Railway authorities inspecting the scene said the derailment was caused by a broken sleeper.
Two of the passengers on the six-carriage train sustained head injuries and bruises. They were identified as Sathit Arsajao, 47, and Arsman Bin Mutae, 31.
The accident forced 14 trains scheduled to travel on the tracks to and from Sungai-Kolok to suspend service. They terminated services at Tanyongmas station in Rangae district while track repairs were under way.
State Railway of Thailand (SRT) said repair workers from Tanyongmas and Hat Yai district, Songkhla, will bring in heavy equipment and begin salvage operations on the derailed train on Wednesday morning.