Most flatcars (US) or flat wagons (UIC) cannot carry more than one standard 40-foot (12.2 m) container on top of another because of limited vertical clearance, even though they usually can carry the weight of two. Carrying half the possible weight is inefficient. But if the rail line has been built with sufficient vertical clearance, a double-stack car can accept a container and still leave enough clearance for another container on top. This usually precludes operation of double-stacked wagons on lines with overhead electric wiring. China runs double stack trains with overhead wiring, but does not allow two maximum height containers to be stacked.
In the United States, Southern Pacific Railroad (SP) with Malcom McLean came up with the idea of the first double-stack intermodal car in 1977. SP then designed the first car with ACF Industries that same year. At first it was slow to become an industry standard, then in 1984 American President Lines started working with the SP and that same year, the first all "double stack" train left Los Angeles, California for South Kearny, New Jersey, under the name of "Stacktrain" rail service. Along the way the train transferred from the SP to Conrail. It saved shippers money and now accounts for almost 70 percent of intermodal freight transport shipments in the United States, in part due to the generous vertical clearances used by U.S. railroads. These lines are diesel operated with no overhead wiring.
Double stacking is also used in Australia between Adelaide, Parkes, Perth and Darwin. These are diesel only lines with no overhead wiring. Double stacking is used in India for selected freight-only lines.