The designation Pb stood for two electric double locomotives built for passenger service on the Malmbanan between Kiruna and Riksgränsen. They were built in 1922 to reinforce the two Pa locomotives. Each section had a two-axle leading bogie and two driving axles. Two traction motors powered a common crank axle that was connected to the driving axles via coupling rods. The bodies had been constructed from wood. With 2,260 hp, they could haul 500 tonnes, while top speed was 100 km/h. Each section carried its own number, so 52 and 53 formed a pair, while 54 and 55 formed the second one.

In 1926 the pair 52/53 received steam heating boilers in both sections and then the sections were switched, what resulted in two pairs with one boiler each. As their adhesion turned out to be insufficient and the Ds was introduced into passenger service, the SJ decided to rebuild them into four 1-C-1 single-section locomotives. Most parts of the locomotives had been built new and their bodies were now made of steel. Each one now could haul 300 tonnes. They were moved to the Malmö region in the forties and withdrawn in 1960.