The Brünigbahn (formerly: SBB Brünig) was, until 31 December 2004, the only narrow gauge railway of the Swiss Federal Railways (SBB-CFF-FFS). On 30 June 2004, the Swiss Federal Council empowered the SBB to sell the Brünigbahn to Luzern-Stans-Engelberg-Bahn (LSE).
The line was constructed by the Jura-Berne-Luzern-Bahn (JBL), which opened the section between Brienz over the Bruenigpass to Alpnachstad in 1888. Initially, the line only provided a connection with steamships on Lake Brienz and Lake Lucerne. In 1889, the Alpnachstad - Lucerne line was opened. With the transfer of JBL to SBB in 1903, year-round operation was introduced. Only in 1914 did the last section between Brienz and Interlaken follow. Electrification took place in 1941 and 1942.
The Brünigbahn begins in Interlaken at a station shared with the Berner Oberland Bahn (BOB). In Brienz the starting point of the Brienz Rothorn Bahn is beside the Brünigbahn station. The Meiringen-Innertkirchen Bahn (MIB) begins with a short stretch parallel to Meiringen station. Alpnachstad is starting point of the Pilatus Railway. The section between Hergiswil and Lucerne is shared with the LSE. On 1 January 2005 the Brünigbahn was merged with the LSE to form a new undertaking, the Zentralbahn.
Brünigbahn, LSE, BOB and MIB tracks are connected, although the latter two use different current systems, so locomotives cannot be shared unless specially equipped. The technology of being able to run electric locomotives or EMU's under different voltages and frequencies and/or under AC or DC has existed for some time now. See Multisystem locomotives.