After reading so many mixed reviews of these trains I was slightly trepidatious on embarking on an overnighter, but thinking it through, the worst that could happen would be a crummy night's sleep, so decided to book a ticket anyhow. If bad, the experience would perhaps be some thing worth writing about, and if good, then great!
The major complaint by most people seemed to be regarding cleanliness and the presence of roaches. Perhaps all the people who have been pleased by the trip haven't bothered to leave reviews then? I was utterly shocked by how clean the compartment I had was, and it seemed all the other compartments in my car were equally clean. There were even flowers in a vase on the table in every room!
Mattresses were typical of Vietnamese mattresses, which is they were firm. I love a firm mattress, so was very comfortable. I had a top bunk, which I found to be wide enough for myself and my laptop and camera to rest. Linens were clean, and there is an open compartment beyond one's feet where additional baggage would be stored if necessary (I kept mine under the bottom bunk, but I suspect I should have tossed it up here). I would recommend packing along a jacket like a Patagonia Nano Puff (warmish, but packs down into a small pocket). The a/c is frigid, and my only complaint, and it goes on and off all night. The linens provided are not really up to the task of keeping one warm.
The train was noisy and rocked a bit, but these are old tracks, so hardly surprising. If you're a European used to the smooth ride of your high-speed rail, this is not it, but if you're an American and have traveled with Amtrak, then this is pretty on par with that.
The toilet was fine, I thought, and the window in there was kept open at all times, so it didn't smell too strongly. I wouldn't envy any ladies, or gents needing to sit though. Just empty your bowels before departure and you should be fine.
After I was the only person left in my compartment a man came along to shoo me out and into another room. It took a second for me to deduce that he wanted to clean, and because other people had departed (this was about three hours to my eventual arrival at 12:30pm) other cabins I could easily be consolidated into one of those. This is quite practical thinking, but being as this is something that wouldn't happen in the States I was initially surprised by it. Great idea though, and the guy gave me a handshake once I understood and moved my things for him.
Anyway, it was a great trip. Scenery closer to Da Nang is gorgeous. Did not buy any food on board, just brought some leftover pizza as dinner (do go to Cugini in Tay Ho, if you're in the area). I waited until I got off the train to get anything else. Hunger doesn't bother me much, and I knew there would be better food in Da Nang than whatever I might find on the train.