Maglev trains ride on a massive metal cable orbiting the Earth. Elevators link the trains with the Earth. In addition to transporting cargo, the trains stabilize the cable and replenish its orbital energy. The contraption may fail catastrophically when the maglev train malfunctions. The minimum mass is 108 tons.
Paul Birch, "Orbital Ring Systems and Jacob's Ladders - I," Journal of the British Interplanetary Society, Vol. 35, 1982, pp. 475-497.
Kenneth A. Brakke, "The Skyrail," L-5 News, July 1982, pp. 6-9.