I suspect it would likely train car dependent.
The four person sleeper car for our train did not have a lot space for luggage. There was a limited amount of space under the bottom bunk for storage. I don't think a thick suitcase would have fit under there and I don't recall any storage space in the upper part of the cabin nor outside our cabin.

We were only going to Sapa for a few days so we left our main luggage at our hotel in Hanoi and packed essentials in our daypacks. If we brought our larger luggage, it would have likely had to just sit on the floor between the bottom bunks or one of the spare bunks I suppose.