I did it again for the second time a few months ago. I do have some pictures which I can email you (or someone can explain how to attach hah). I went up on the Chinese carriages and came back on the older ones. The Chinese ones were wonderful. Modern, bright and clean. They have Wifi in the restaurant car and serve much more than the older ones in the way of food. The older one coming back is made of metal. I mean everything is metal. Its fine and will do the job, but when you look down the carriage everything is metal, the beds, the ladders, the luggage racks, makes it quite interesting. But it is old, and the toilets and other facilities reflect that (I wouldn't have fancied being a woman and needing the toilet, put it that way). I found you get more locals on the Thai ones and more tourists on the Chinese ones. I'd say take the Chinese ones every time if the timings and budget allow it, they are really good.
My only (I don't want to say criticisms because its down to personal taste really) pointers would be (apologies if you have been here before)
1. The air con is really cold. Now this is fine for me, because I feel the heat, so I love it. But I know plenty that that needed a jumper on the sleepers.
2. The staff on the Chinese carriages kicked you out the restaurant/wifi/social car at midnight. After that it was beds only, so bear that in mind if you want food/you're in a group.
3. I'd always recommend booking bottom bunk as the top bunks have no window on either design of carriage. Also, if you're solo, and the bottom bunker wants to sleep, you have no choice to go up top with no window. Not a problem, but if you fancied watching out the window, bear that in mind too.
I also used 12GoAsia and thoroughly recommend them. I had a journey with them carrying into Malaysia cancelled due to the terrible flooding in Southern Thailand in January, and I had a full refund within 24 hours and had no issues booking further tickets with them later on during my trip. The office in Bangkok speaks English incredibly well. So yeah, 100% recommend them.
Any other questions, please feel free to ask. I'm getting pretty knowledgable with Thai State Railways haha