Took the overnight train Bangkok to Chiang Mai, loved the experience. Booked 2nd class with 12Go all went well with the booking. Top tips, eat before boarding, take something warm to sleep in, bring your own breakfast, eye mask, earplugs, wet wipes and hand sanitiser. The toilet is an experience in itself.
Would definitely recommend taking the opportunity to ride the train at night and wake up to rice fields and rural life.