The A class of the Swedish State Railways (SJ) was a type of steam locomotive built in 1906–1909 for hauling mainline express trains. The 26 locomotives were built to replace older types that could not cope with the increasingly heavy express trains, but soon became insufficient themselves. They were relegated to less important passenger trains, and five were transferred to then-independent Ostkustbanan (OKB) in the 1920s, but returned when that company was nationalized in 1933.
OKB and SJ had a total of eight locomotives rebuilt, replacing the trailing wheels with an extra pair of driving wheels. The locomotives that had not been rebuilt were scrapped in the late 1930s, except for number 1001 which is preserved at the Swedish Railway Museum, but the rebuilt ones survived until the early 1970s. OKB also built two locomotives of a related design, but with a smaller driving wheel diameter. These were also taken over by SJ and were in regular use until 1961; one has been preserved.
The A class locomotives were the first of SJ with superheaters, and have been described as the first modern steam locomotives in Sweden. Some parts introduced with the A class were to become SJ standard, fitted to several types of locomotives. The tender designed for the A class was also used for the much more numerous B class.