I did a cab ride in UP 844 from Cheyenne to Denver a few years back. It's a very smooth ride since the track is all welded rail and excellently maintained roadbed. The noise level is actually quite moderate but if course ear plugs are mandatory anyway.
I rode diesels quite a bit and there you are much closer to the front than in a steamer. But the feeling is of mass gently rocking and moving. Even when you go into emergency it's not a jarring event. But get on to rough track and it gets easy to get thrown across the cab.