The D, introduced in 1925, was the first standard electric locomotive of the SJ to be built in large numbers. It had a 1-C-1 wheel arrangement and was powered by two traction motors that drove a common crank axle through coupling rods. In the early batches, each motor developed 610 kW, and the locomotives featured wooden bodies with gangway connections at both ends. The freight version, geared for a maximum speed of 75 km/h, was designated Dg, while the passenger version, geared for 100 km/h, was designated Ds.
After the first 134 locomotives had been completed, construction changed from wood to steel in 1933. The original locomotives were subsequently classified as D (I), while the newer steel-bodied locomotives became D (II). During later overhauls, the cabs and other structural components of the D (I) locomotives were also rebuilt in steel. Beginning in 1936, more powerful traction motors rated at 735 kW each were fitted. By the end of production in 1943, a total of 333 locomotives had been built, including nine for the Bergslagernas Järnvägar (BJ) and three for the Dalslands Järnväg (DJ), all of which later became part of the SJ fleet.
Following the introduction of the Da in the 1950s, the D locomotives received the same 920 kW motors. This increase in power made them capable of hauling freight trains even with the 100 km/h gearing, rendering the distinction between freight and passenger versions unnecessary. They were therefore redesignated Du, the “u” standing for “universal”. Between 1967 and 1976, 162 locomotives were rebuilt as Du2 by removing the gangway doors and installing multiple-unit control connections in their place. This allowed two locomotives to operate together, providing approximately the same power as the newer Rc locomotives in multiple. Withdrawals had already begun in 1967, and the last examples were retired in 1988.
