What is a 'sleeper car' on a train?

What is a 'sleeper car' on a train?

If you are taking an overnight train, you might want to book a cabin that has a berth, which is a sleeping berth that actually pulls down so that the kids can sleep on the train. I remember doing that when I was a kid, and I thought it was so exciting. It's a separate compartment - sometimes it even has a little bathroom - and if it's a very fancy train, you have your own little bathroom. But it has berths that come down, and you sleep on the berths. They have both high and low berths, so you can actually book - there are singles, there are doubles, and there are quads.