Old Main Line is a hiking, biking, and horse trail in Ellicott City, Maryland. It is within Patapsco Valley State Park: Daniels Area and Patapsco Valley State Park: Woodstock Area. It is 2.3 miles long and begins at 258 feet altitude. Traveling the entire trail is 4.5 miles with a total elevation gain of 296 feet. Along the trail there are a beach and a parking. The trail ends near the Camel's Den cave entrance.