I copied this from the trails.com web site. It sounds like the trail is flat, not very challenging, and about 3 miles long.
Preview: Just over 2 feet deep and perhaps 20 feet wide in spots, Capoolong Creek meanders through central Hunterdon County before emptying into the South Branch of the Raritan River. An aerial view would show it as a pretty ribbon winding through several rural communities. Running along the creek from Pittstown to Landsdowne is a long, narrow wildlife management zone marked with numerous posted signs. The Capoolong Creek Wildlife Management Area, one of 126 WMAs in New Jersey, serves as a buffer zone to development. The wildlife management area encompasses what was once a branch of the Lehigh Valley Railroad, a fact that is evidenced by lingering railroad bridges, embedded ties, and even a train station, now in disrepair. The rail trail is a little over 3 miles long, relatively flat, and quite lovely as it winds along beside the creek. This is not a challenging ride by any means, but the scenery makes it worth the effort.