Phetchaburi police said the bomb was placed under the rail track, 680 metres from the Ban Cha-am station.
The explosion occurred at 3.50am cut the rail track and damaged two tracks. The blast left a 40 cm deep hole at the foundation of the track.
Three trains had to wait at Hua Hin station in Prachuap Khiri Khan, awaiting the track repair.
It was expected that services would resume at around 10am.