The first stop on the southern line where the coast opens out โ a provincial capital with a bay, a monkey mountain and almost no tour groups.
Find a room before you go.
Prachuap Khiri Khan is about three hundred kilometres south of Bangkok, and the train is the obvious way to do it: roughly five hours on the southern line, with four services a day and a fare that costs less than a taxi across Bangkok. The line runs inland at first and then swings towards the water, so the last stretch is the good bit.

The station sits in the middle of town, ten minutes' walk from the seafront. That is the part people miss when they compare it with the bus, which drops you at a terminal on the edge of things.
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Four daily services on the southern line. Second class with a fan is fine for a daytime run; the sea appears on the left about an hour before you arrive.
Departures through the day from Sai Tai Mai, Bangkok's southern terminal. Faster than the train when the road is clear, and less pleasant when it is not.
Straight down Phetkasem Road. Hua Hin, Pranburi and Sam Roi Yot are all on the way, which makes this the one to pick if you want to stop.
Step-by-step transport guides for this stretch of the Gulf coast.