The U was an electric shunter developed by ASEA for the SJ in the twenties, parallel to the D. The three prototypes built in 1926 had three axles, powered by one 515 kW traction motor via a crank axle and had their cab at one end. The pantograph was mounted on top of the cab roof. Later they were designated Ua to distinguish them from later variants. Since the end cab restricted the view in one running direction, later locomotives were built with a central cab.

The main production variant was the Ub of which a total of 123 were built between 1930 and 1951. Only 90 of these went to the SJ. 15 more were built as LKAB type F and five more went to TGOJ. Additionally, 13 went to Norway as NSB El 10. Originally the traction motor still delivered 515 kW, but later they got the more powerful motors with 610 kW which were also used by the D.

The Uc was a single locomotive built in 1933 that was modified for use with an additional battery trailer. It could only deliver 215 kW on battery power and was later only used under overhead power like all of its sisters. More success had the Ud, of which 25 were built in 1955 and 1956. It had an additional crank axle that improved mass balancing and allowed to increase the top speed from 45 to 60 km/h. It also got roller bearings and improved insulation for the cab. The original traction motor had 620 kW and was later exchanged for a new one with 720 kW.

In 1987, the SJ started to rebuild their locomotives. 60 Ub were rebuilt into the Ue which could be recognized by a new grey, blue and yellow livery that was later changed to red-orange and dark blue. They had received multiple controls, remote control, central lubrication and a new 920 kW traction motor. From the Ud, 16 were rebuilt in a similar way, which were now designated Uf.