When the Bergen Railway was converted for higher axle loads, more powerful locomotives than the type 26 could be used there. This resulted in the type 31 with the same axle arrangement, but with a higher weight and boiler pressure. In 1915, two locomotives were initially built which, like the types 26a and 26b, had a four-cylinder engine with simple expansion.

Locomotive Magazine, August 1917

The high tractive power that could be achieved for a short period of time by this engine was used to push snow blowers on the Bergen Railway. In 1920, two more identical locomotives followed for the same purpose, which were also supplied by Thunes from Oslo and were designated together with the first two as type 31a.

Sectional drawing

Locomotive Magazine, August 1917

As with its predecessor, the majority of locomotives were built with compound engines in order to be able to maintain a similarly high tractive effort over a longer period of time. These locomotives, which were designated as Type 31b, were supplied by various manufacturers between 1923 and 1926 and took over the tasks of the type 26. They thus pulled different train types on the Bergen Railway until the Nohab diesel locomotives of the Di 3 type were introduced there in 1957 and 1958.

They were now distributed to other locations where there was no electrification or where diesel locomotives were not yet available. They spent their final years on branch lines that could withstand the weight of these locos. Only the four type 31a locomotives remained in service until the end of their lives in their traditional role as pusher locomotives for the snow blowers and were not put out of service until 1970.