The U.S. standard railroad gauge, the four-foot, 8.5-inch distance between rails, followed English practice because English expatriates designed U.S. railroads. English railways had inherited that gauge from earlier tramways, whose builders used the same jigs and tools they had used for wagons. Those wagons used that wheel spacing because other spacings would fail in the ruts of England’s old long-distance roads. Those roads originated with Imperial Rome, whose legions built the first long-distance roads in Europe, including England, and whose war chariots formed the original ruts that later vehicles had to match. In this account, the standard railroad gauge ultimately traces back to the width of Imperial Roman war chariots, said to be just wide enough for the rear ends of two war horses. The same chain of constraints is then linked to the Space Shuttle: its solid rocket boosters were made in Utah and shipped by train through a mountain tunnel, so their width had to fit rail transport limits shaped by that gauge.