In 1883, miners seeking to transport the mineral borax from mines in Death Valley to the nearest railroad spur, 165 miles (266 km) away in Mojave, California, came up with a novel solution: massive wagons hauled by teams of 18 mules and two horses. When fully loaded, the caravans weighed 36.6 tons (33.2 tonnes) and stretched over 100 feet (30 m). The teams hauled more than 20 million pounds (9,000 tonnes) of borax over a six-year period. Why were two horses included on each mule team? Discuss
Source: The Free Dictionary