My advice is to take the 2nd class aircon sleeper but book at least 2 weeks in advance to secure a lower berth. The lower berth is wider than the upper, has a picture window to admire the Thai countryside, and does not have the aisle ceiling lights glaring over the top of the curtain preventing your getting to sleep, but more importantly you have room to bring your luggage into the sleeping area behind the curtain. Leaving your luggage outside all night is too tempting for thieves. Oh, and watch out for the little olde Thai ladies who will claim your lower berth the second it is made up by the porter. Hot air rises as a rule, so it gets overly warm and hummy in the upper berth. Have always travelled 2nd class aircon, but only lower berth after a miserable night 16 years ago crammed into an upper berth. For what it's worth.... Thai train travel has got to rate worldwide as value for money. INHO the 1st class roomettes are akin to jail cells so I avoid them. Cheers...