Train 85 from Hua Lamphong Station, Bangkok to Hua Hin beach resort, is an evening Express service offering a range of seating from cheap 3rd class, to 1st class.
3rd Class seats - fan cooled carriage:
2nd Class seats - fan cooled carriage:
2nd class day and night carriages (air-conditioned).
1st class day and night carriages (air-conditioned).
For this relatively short journey, we would recommend you travel in the 2nd class seat only carriages, or if you are on a budget, the 3rd class seat carriages.
2nd class seats are quite comfortable and recline, plus the carriage is much less crowded than the 3rd class coach.
If you want air-conditioning, you will have to pay quite a bit more to travel in one of the 2nd / 1st class Day & Night carriages.
These are much more expensive as you will be paying for a bed.
If you want to travel in any of the 2nd class coaches, it is always best to book your tickets in advance as spaces can be quite difficult to get during busy periods of the year.
If you want to travel in 3rd class, you should have no trouble obtaining a ticket on the day of departure.
To book train tickets in advance in Thailand you can:
1) Visit any SRT railway station ticketing counter or advance booking office (in larger stations).