Double-stacked rail transport, where containers are stacked two high on railway cars, was introduced in the United States. The concept was developed by Sea-Land and the Southern Pacific railroad. The first standalone double-stack container car (or single-unit 40-ft COFC well car) was delivered in July 1977. The 5-unit well car, the industry standard, appeared for the first time in 1981. Initially, these double-stack railway cars were deployed in regular train service. Ever since American President Lines initiated in 1984 a dedicated double-stack container train service between Los Angeles and Chicago, transport volumes increased rapidly.