If you bring a rail, it is possible to put one up - but the mattress is not that heavy, so be careful that it will stay in place once set up.
It is about a foot and a half off the floor when opened, it opens out and rests on top of the bench part of the built-in.
I actually spent a couple of nights on it during our trip in November, and while it is too short for most adult people it isn't that bad. Comfort-wise I actually find it a great improvement over the old trundles, which I had also tried over the years.