Hulbert, Edward Phinney,

Gravity railroads preceded steam locomotives by about a decade. The term refers to cars moving along wood and iron tracks, descending down inclined planes only by the forces of gravity and returning by a cable or pulley system. They were used most notably in the transportation of anthracite coal from northeastern Pennsylvania mountain tops. Initially, horses or mules powered the return trip, which were later replaced by steam engines. The earliest gravity railroads had single tracks, which limited traffic, and rope or hemp cables that broke frequently.

From the description of Pennsylvania Coal Company : Photographs of the Gravity railroad near Scranton : photographs, 1950. (Princeton University Library). WorldCat record id: 213343256