Originally Posted by rm -rf

"Crushed stone" rail trails are often hard packed and smooth. The crushed stone packs down hard, with a very thin layer of looser bits on top. It's nothing like gravel or sand.

Narrow tires work, but wide smooth tires float over the surface with pretty low rolling resistance. Perfect for these rail trails.
There can be occasional deeper loose spots or gravel temporary repairs. The smooth tires are still okay on those.

Fenders will keep your bike cleaner from dust or a damp surface layer.

From the Pennsylvania GAP trail: