Special express train No 36 with nine carriages had departed Butterworth station in Penang state of Malaysia for Bangkok. The fifth carriage, which was carrying freight, went off the track near Khao Tamone station in Phetchaburi, State Railway of Thailand (SRT) governor Prapat Chongsanguan said.
There were no injuries, but the derailed freight car blocked the track. Passengers in trailing carriages were moved up to the first four carriages and the train continued to Bangkok.
The area where the train derailed was the responsibility of a contractor tasked with track maintenance, he said.
Mr Prapat said the contractors had stacked sleepers and other equipment on the shoulder of the track. As the train passed the spot, some of it slid onto the track, causing the derailment.
He said the SRT had warned the contractor and would bill the firm for expenses incurred by the derailment.
Mr Prapat said he has a schedule to examine the whole southern track in January, before making a decision whether to suspend all services to the South to allow major maintenance, like the SRT earlier did with the northern route.