Gravity railroad

A gravity railroad (American English) or gravity railway (British English) is a railroad on a slope that allow cars carrying minerals or passengers to coast down the slope by the force of gravity alone. The speed of the cars is controlled by a braking mechanism on one or more cars on the train. The cars are then hauled back up the slope using animal power or a stationary engine and a cable, a chain or one or more wide, flat iron bands. A much later example in California used standard gauge steam engines to pull gravity cars back to the summit of Mt. Tamalpais.

The typical amusement park roller coaster is designed from gravity railroad technology based on the looping track incorporated into the second railroad of the United States, the Mauch Chunk & Summit Hill Railroad, which remained in operation for decades as a tourist ride after it was withdrawn from freight service hauling coal.

Some gravity railroads were designed to allow the weight of the descending loaded cars to lift the empty cars back up to the top, using a cable looped around a pulley at the top for a portion of the line. A later revision designed by John B. Jervis, used two separate tracks known as the loaded or heavy track which carried cars loaded with coal to the destination, and the light track, used to return empty cars to the mines. This method allowed cars to travel in a loop, without the need for passing sidings. A stationary steam engine and a looping cable, chain or iron bands were used to raise the empty cars up the lift planes. The cars then coasted down a slight grade to the next lift plane. When cars reversed direction at the ends of the line on a switch or turnout instead of a loop, the railroad was known as a switchback gravity railroad.

The term "switchback gravity railroad" is sometimes applied to gravity railroads that used special self-acting (momentum-driven) Y-shaped switches known as switchbacks to automatically reverse a car's direction at certain points as it descends; this essentially folds the incline across the slope in a characteristic "zig-zag" shape. (See diagram: car starts from point A, coasts through switch at B, and comes to a stop at C. Car then rolls through the switch again and proceeds to the switch at D, where the process is repeated.) A separate track was typically used to haul the empty cars back to the top.

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