Simply, a cable car is a vehicle pulled by a cable. They are used when the grade is steep or when there are obstructions to be traversed such as canyons or rivers. A rail cable car is run on supporting rails and pulled by a cable. Cable cars in San Francisco utilize a continuous cable that moves in a groove below the rails, a system invented and patented in 1867 by Andrew S. Hallidie. Each car has a gripping device that can be clamped onto the moving cable or released from it. There also are regular and emergency brakes. Aerial cable cars are suspended from overhead cables supported by towers. Any cable railroad that traverses a steep grade may be called a funicular. More specifically, however, a funicular has two cars connected to opposite ends of a cable, with the cable looped around and powered by a pulley system at the top of the grade.