The narrowest part of Thailand, crossed in an afternoon: from the Gulf coast to the Andaman Sea, and from one weather calendar to the opposite one.
Find a room before you go.
This is the short way from one sea to the other. Chumphon sits on the Gulf; Ranong sits on the Andaman coast facing Myanmar, and the road between them crosses the narrowest part of Thailand in about two and a half hours.

The thing to know first: Ranong has no railway station. A ChumphonโRanong line has been proposed for years but nothing runs. If you are coming by train you get off at Chumphon and continue by road, and Chumphon is the nearest railway station to Ranong, full stop.
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The usual way. Vans leave through the day from Chumphon and will normally pick up near the railway station if you ask when you buy the ticket.
Slower and roomier than the van, with departures from the Chumphon bus terminal. Worth it if you have luggage.
Highway 4 straight across. The road climbs through forest at the watershed, which is the best part of the drive and the reason Ranong gets so much rain.
Step-by-step transport guides for this stretch of the Gulf coast.