The Frederick Branch is a railroad line in Frederick County, Maryland. It was built by the Baltimore and Ohio Railroad (B&O) in 1831, and is now owned by the Maryland Department of Transportation (MDOT). The 3.4 mi (5.5 km) branch extends between Frederick Junction – a wye with the Old Main Line Subdivision of CSX Transportation on the west side of the Monocacy River – and its terminus at East Street in downtown Frederick, Maryland. The wye at Frederick Junction was the first example of its kind in the United States and is still in use today.