METRO began running light rail service (METRORail) on January 1, 2004. Currently the track runs approximately 13 miles (20.8 km) from Downtown Houston to the Texas Medical Center and Reliant Park Southbound and to the Northline Mall Northbound. METRO operates an extensive park-and-ride bus system to serve many of Houston's outlying suburban areas. Most of the park-and-ride buses run in barrier-separated high-occupancy-vehicle (HOV) lanes that provide direct service from park-and-ride parking lots to major employment destinations. Prior to the opening of METRORail, Houston was the largest city in the United States without a rail transit system.