It depends on the train. If you're on the Texas Eagle/City of New Orleans, you're directly behind the locomotive, so the horn is quite loud. I had earplugs in and headphones on and still didn't get much sleep.
I've been on other trains that have two locomotives and a baggage car ahead of the transdorm. It's amazing what having two cars between the lead locomotive and the transdorm can do for noise. I still needed earplugs, but that's because I'm a ridiculously light sleeper. I could just barely hear the horn when I had my earplugs in. It was hardly noticeable, sort of like a train horn off in the distance when you're sleeping at home.
