The Montour Trail is a non-motorized trail located in the vincinity of Pittsburgh. It is currently comprised of several useable but unconnected sections of trails between Coraopolis and Clairton. Each year existing trails are extended and new sections added, with the eventual goal of forming one continuous 47 mile trail.
The trail surface is usually paved with crushed limestone, with some variations depending on what part of the trail you're at. The trail is ideally suited for bicycling, walking, jogging, cross-country skiing, etc. In certain portions of the trails, horses can be ridden.
More information can be had at the trail organization's homepage:
http://www.montourtrail.org/