The San Diego and Arizona Railway (reporting mark SDA) was a 148-mile (238 km) short line U.S. railroad founded by entrepreneur John D. Spreckels, and dubbed "The Impossible Railroad" by engineers of its day due to the immense logistical challenges involved. It linked San Diego, its western terminus, with El Centro, its eastern terminus, where passengers could connect with Southern Pacific's transcontinental lines, eliminating the need to first travel north via Los Angeles or Riverside.