The Imperial was a night train of the Rock Island and the Southern Pacific.

It operated from Chicago to Los Angeles, via Kansas City (Missouri) and Tucumcari (New Mexico).

Among the Southern Pacific passenger trains running from Arizona to southern California, it was distinctive for stopping in Mexico and for having an international branch running into Mexico a second time, stopping between Calexico and San Diego.

Heading west of Yuma it went into Mexico for two stops, then returned to Calexico; there, the main route continued to Los Angeles.

A southern branch split from the main route at Calexico on the San Diego Short Line.