With the construction of the Bergen Line, which was completed in 1909, it was decided that there would be built branch lines to two fjords, the Hardanger Line to Hardangerfjord and the Flåm Line to Sognefjord. Both branches were steep and curvy, which set high demands on the locomotives. The Hardanger Line was long, had a maximum gradient of 4.5 percent, a minimum curve radius of , and a maximum speed of . The Flåm Line is long, had a maximum gradient of 5.5. percent, a minimum curve radius of and a maximum speed of uphill and downhill. Both lines had a maximum permitted axle load of , standard gauge and a electrification system. The Hardanger Line opened in 1935 and the Flåm Line in 1940, although the latter did not receive electric traction until 1944.