Follow the Switchback trail (white) downhill and then along the river. Trail is a mixture of singletrack and doubletrack. Last 2/3rds of a mile is on Plantation Trail (Red) and mostly uphill. This is a great trail to push the grade from beginner to intermediate. Good downhill to begin, some challenging (to me) singletrack on the side of a slope and a short technical rocky descent (again challenging for beginners/intermediate). But there is also plenty of smooth singletrack and doubletrack next to the river to get your breath back. Quite a big climb near the end. I've ridden the trail a few times and have had to put my foot down each time. Still there's always next time.
Take I-70(W) then exit 83 for Marriottsville Road North. Follow this for about 4 miles then turn right into McKeldin Area. Go up the hill and park behind the ranger station.
http://www.singletracks.com/item.php?c=1&i=2827