The company I work for is a railroad contractor and we do work on 6 of the 7 clsss ones, as well as a number of short lines. I’ve traversed thousands of miles of track over the years.
It’s true that the majority of crossings are “unprotected” (no gates or flashers). It costs a ton of money to outfit a crossing with gates or flashers and to maintain it. There will always be lots of rural crossings that only have a crossbuck sign.
As for speeds it’s really more dependent on the engineering of the track and geography of the area than just population proximity. There’s lots of track out there that’s 60 mph freight and 80 mph Amtrak very close to heavy populated areas. Of course there are also plenty of speed restricted areas through major metropolitan areas.
Engineers get into deep **** if they’re caught speeding even 1 mph over the max speed for that segment of track. Most likely taken out of service immediately. Also, if they’re traveling significantly slower than track speed for some reason, the dispatchers will get on them pretty quickly.