The A class of the Swedish State Railways (SJ) was a type of steam locomotive built between 1906 and 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.

[2]: 80  These locomotives were too small when trains became heavier, particularly after bogie carriages were introduced by SJ in 1891, and double heading was often needed.

[3] SJ started working on a new fast locomotive that was to be capable of hauling seven carriages with an average weight of 30 tonnes (30 long tons; 33 short tons) in 90 kilometres per hour (56 mph) on flat ground and in 50 km/h (31 mph) on long 1 in 100 gradients.

[6]: 119 At this time, the Swedish railways had a general speed limit of only 90 km/h, but the A class was considerably faster.

[1]: 35  Number 1001 was allocated to the Swedish Railway Museum in 1937, and has been on display since 1942 with some parts cut open to show how a steam locomotive works.

[1]: 268 The railway between Gävle and Härnösand was built by the private company Ostkustbanan (OKB), the first section opening in 1923.

[7]: 13  The company was troubled financially from its inception, and depended on loans from the state;[7]: 11  it was controlled by the Swedish government from 1924.

[7]: 20 OKB also found the performance of the A class unsatisfactory, and had them rebuilt at Motala Verkstand in 1929 to a 4-6-0 wheel arrangement.

[7]: 20  OKB was nationalized in 1933, and the locomotives were returned to SJ where they regained their old numbers but were given the new A3 class designation.

[1]: 37  Most of the locomotives were again given C class tenders for some time, and several were modified for wood-firing during World War II and equipped with smoke deflectors of the Wagner type.

[1]: 36  They became the A class once again when many locomotive types were given new designations in 1942, and remained on the East Coast Line until electrification, when they were placed in the strategic reserve.

[2]: 65 The strategic reserve was made up of surplus steam locomotives, preserved in usable condition to replace diesel-powered vehicles if the import of oil was interrupted.