The directly owned main line of the Bern-Lötschberg-Simplon railway ran from Thun, where it connected with the Swiss Federal Railways through Spiez and the Lötschberg Tunnel to Brig, where it connects with the Swiss Federal Railways owned Simplon Tunnel to Italy. The route between Thun and Spiez formed part of Lake Thun line, which also serves a terminus at Interlaken, whilst from Spiez to Brig the Lötschberg line was used.