Taking an over night train in Thailand is ok, I was able to sleep in the second class. It's not great luxury but it's doable. Often the best quality-price ratio taking into account time needed for the trip. Don't expect any services other than toilet. In all the stops people though come to sell food but you can also take your own picnic or eat before/after getting in the train. In the night train you get clean white sheets and a curtain to have some privacy!
I've always had great experiences on the trains in Thailand, I love them. This time I booked the sleeper bed (I usually buy 2nd class sleeper fan which I hundred per cent recommend). First of all the train was 1h 30 min late, the bed carriage felt like a fridge: cold and humid and it smelled weird, like humidity. The beds felt moist because of the humidity in the air. Needles to say I barely slept. So I payed extra for "sleeping" in a poorly ventilated fridge. The 2nd class seater fan carriage is way better.
Lots of noise every stop so you wake up about every hour and from about 5 am almost every stop there were vendors boarding the train and trying to sell stuff.
Bathroom was really stinky too
Besides that the bed was fine and we got a clean mattress and pillow with a blanket and the staff and conductors were nice :)
Train was 1 hour late and arrived 4 hours late to Bangkok. Sleeper seats ok, nice to be on horizontal position on a 12 hours trip. Quite noisy night but exciting experience for all of us. Maby train 170 is a slower- one. Train was full so we got sleeper beds around one van. Kids are already teenagers so this wasn't a problem.
2nd class night train does the job, but I'd recommend anyone to book 1st class if they can for a better night's sleep! Booking on 12goasia and receiving the PDF tickets in minutes made the whole trajectory a lot easier.