Railway officials are attempting to identify a large piece of metal that came close to derailing a Bangkok-bound train in Prachuap Khiri Khan early Tuesday.
All 500 passengers from Yala and Surat Thani were transferred to vans and a second train to continue their journey.
Train driver Prasit Molee was slowing for the approach to Kui Buri station at 2am when he spotted the chunk of metal, but he could not stop in time to avoid rolling over it.
The metal caused severe damage to the braking and air-conditioning systems, effectively putting the six-carriage train out of service.
Prasit said if the train had not been slowing, it could have been derailed.
Officials are trying to determine the origin of the metal amid speculation it could have fallen off an earlier train or been intentionally placed there in a sabotage attempt.