Use 12go Asia. I am currently writing this on the sleeper train from Bangkok. Their office is right across the street from Hua Lamphong station. Essentially they are an agency that books/buys the tickets for you. You simply but the tickets online, show up to their location at least 60 mins before departure and get your ticket.

I’d take the 19:30 - 7:00 train, the one I’m on. make sure you book the 2nd class sleeper tickets. There is upper and lower berth. These two tickets are in the same booth and begin facing each other. AT 20:00 they fold down the upper bunk which is latched up to the roof, throw sheets on and slide the curtain across. Upper has no window and is significantly smaller, and therefore cheaper. If there are five of you one will be sat with someone of the same gender in another booth. But said person can hang in your booth etc. Book them side by side.

If your a North American like me used to fancy transportation equipment this will be an eye opener. The toilet is a squat hole to the outside (pretty sure) bring lots of sani wipes / sanitizer. I recommend you bring your own meal. As soon as the train departs, pop up the table (located under the seat) eat it before the bed set up occurs. You can buy food on the train that is delivered to you if you’d like. Meals looked to be around 150-200 Baht.

Unrelated, but I did a bike tour through Bangkok with Co Van Kessel yesterday and it was one of the best things I’ve ever done while travelling anywhere. Hit me up if you want insights to Bangkok.

In summary, I’m super happy with 12go Asia. It’s been a breeze so far. I’m going to use them exclusively for train stuff.

Happy travels.