Michaux State Forest features 20 miles of hiking trails near Chambersburg, PA.
Take Route 30 east from Chambersburg or west from Gettysburg. Take 233 north (Pine Grove Road) at a traffic light. After about 2 miles take a left onto Shippensburg Road. Drive up the hill and there will be a snowmobile trail lot on the left (about 1.5 miles). Park and hit the trail. I usually ride down the fire road, take a left onto the powerline trail, then hang a right onto a great downhill trail.
http://www.tripleblaze.com/trail.php?c=3&i=1339