Complex sentence: Another branch line, from Sira to Flekkefjord was once part of the main line, upon completion of Sørlandsbanen in 1944, it was turned into a branch line, it was closed in 1990.
Split into simpler sentences: Another branch line, from Sira to Flekkefjord was once part of the main line. Upon completion of Sørlandsbanen in 1944, it was turned into a branch line, and it was closed in 1990.