In the early years of American railroading, link and pin couplers were used to join cars together into trains. This type of coupler required the brakeman to physically get between the cars while they were being moved, making them incredibly dangerous.
Starting with the invention of the Janney coupler in 1873, railroads in the late nineteenth century gradually began adopting much safer automatic couplers. Ever since 1900, all railroad cars used in interstate commerce in the United States have been required to used automatic couplers. The “knuckle” couplers used on American trains today are directly descended from Eli Janney’s patented design.
Although this change made a significant improvement in railroad safety, it also created a few difficulties. One problem was that cars were closer together using Janney couplers than they were with link and pin. This made it difficult or impossible to navigate tight curves.
One solution to this problem was to an additional auxiliary coupler between cars. The July 1902 issue of Railway and Locomotive Engineering has a couple of pictures of this device here (scroll down to page 330).
(h/t to Darrell Smith, who posted this information on the Early Rail mailing list.)
