The problem with machinery and trail maintenance is that machinery is heavy, and unless it can wheel itself to your worksite under its own power, it's unlikely going to save you any labor.
It's also straight-up illegal to operate any sort of vehicle in US wilderness areas (horses are OK). I think they also ban gas motors and maybe even things like electric chainsaws.
From what I've read, there are some areas where the Forest Service/Park Service are allowed to use power tools for maintenance activities. This seems like the exception rather than rule, however.
I have a friend who does trail maintenance in the Dolly Sodds and surrounding areas, and he has gotten quite adept at using a crosscut saw with his partner. I've seen it in action, and the efficiency of that tool, in the hands of skilled users, is quite impressive.
True, but there are plenty of wilderness areas, and the Forest Service needs effective tools for maintaining them. This means crosscut saws and pack mules, sometimes.
USFS manages National Forests that aren't wilderness areas. They do timber sales and such where the logging will be done with machinery and trucked out.
National Park Service manages lands that are legally classified as wilderness areas.
