Comfortable and comfortable. It's certainly much better than spending a night on a seat. But most people won't sleep well. It's noisy and either too hot or too cold. The bed is hard, narrow and often too short for tall people.

I have spent many nights on trains in various countries, seats, floor and berth. I will prefer a couchette or sleeper if the price is somewhat reasonable. But I won't live in the expectation to get something comfortable. Just less uncomfortable.