Train 169 from Bangkok to Hua Hin beach resort is a cheaper Rapid service with 3rd and 2nd class fan cooled carriages, as well as 2nd class Day & Night coaches.
3rd Class carriage seats (fan).
2nd Class carriage seats (fan).
2nd class day and night coach.
Whichever class you choose, the seating is comfortable enough for this fairly short trip.
3rd class can get a bit crowded at times, but is an interesting way to travel.
2nd class seat carriages are a bit more expensive, but more comfortable.
You don't really need to pay the extra for the Day & Night coaches (unless you need air-conditioning), as you are paying for a bed you will not use.
If you want to travel in any of the 2nd class carriages, it is always best to book your seats in advance.
You can book advance tickets from any SRT railway station ticket office / counter.
Tickets on this route go on sale 30 days in advance.