[1] Six prototype trams were delivered by Strømmen in 1937, with four different motor solutions, from AEG, Siemens, Vickers and Norsk Elektrisk & Brown Boveri (NEBB).

The latest series, the HaWa Class, had been delivered during the early 1920s and were, by the late 1930s, becoming old fashioned, with low speed, uncomfortable interiors and a two-axle wheel arrangement.

In 1935, an agreement was made with Stømmmens Værksted to build a series of aluminium-bogied trams and buses.

[2][3] The use of an aluminium body was controversial, and international experts recommended that the tram company should not choose that solution.

The goal was to decrease the weight so the trams could operate with less-powerful motors, giving a lower cost.

The first prototype was a full-scale model of a tram, built at Homansbyen Depot in 1935, using an undercarriage from disused horsecars.

According to tests by JG Brill Company, which was a specialist on high-speed interurban railcars, streamline construction would afford a power reduction of 17% at as low speed as 32 km/h (20 mph).

The various experimental configurations fitted to the prototype trams did not work too well; and instead a conventional motor solution was chosen.

The Strømmen series was delivered from February to June 1939 and leased to Oslo Sporveier's subsidiary Bærumsbanen, which gave them the designation Class B.

The trams partially ran through a relatively rural landscape, and more than any other Norwegian tramway this line may be called an interurban.

[10] During World War II, the rationing of petroleum fuels gave a boost to ridership on the electric tramways.

Lack of parts, particularly for the British Vickers motors, caused long waits for defective vehicles at the depots.

[10][11][12][13] After the end of the war, Oslo Sporveier decided to transfer all the remaining trams to Bærumsbanen.

Their high speed made them well suited on the light rail, but the centre door reduced their capacity.

The order of the SM53-series from Høka gave Oslo Sporveier sufficient stock to operate the street lines, and from 1952 to 1957, the company transferred the 19 trams to Bærumsbanen.

On 2 August 1958, five people perished and 17 were injured in the Strømsveien tram fire, the worst disaster in the tramway's history.

[16] In 1976, a class B tram was retired after it had lost braking power and crashed near Sjømannsskolen on the Ekeberg Line.

This gave a light body that was strong as steel, giving what at the time was regarded as a more elastic structure which would not break as easy.

The centre beam between the two bogies was made of steel, and was intended to compensate for the soft aluminium structure; these were prone to rust and needed repeated replacement.

The bodies had a very streamlined shape and a distinct tail, that in addition to the aesthetic purpose was chosen because it strengthened the structure.

From 1970 to 1974, the trams were rebuilt to remove the conductor, and the driver was given space for a purse and ticket machine, as well as an announcement system.

The bogie frames were fastened to the axle box via a revolute joint and a spiral spring.

187 was equipped with a SV41-bogie from Strømmen; it had a different spring system that allowed the tram to remain at the same height independent of the weight.

[25] This caused problems because the monomotor required the wheels to be ground regularly to avoid uneven driving, but this was not discovered until 1941.