It opened in stages between 1888 and 1916, and was, between 1903 and 2004, the only narrow gauge line of the Swiss Federal Railways.

[2][3][4] The line was constructed by the Jura–Bern–Lucerne Railway (JBL), which opened the section between Brienz over the Brünig Pass to Alpnachstad in 1888.

On 30 June 2004, the Swiss Federal Council empowered the SBB to sell the Brünig line to the Zentralbahn company, formed by the LSE, and which now owns both railways.

The tunnel replaces a less direct surface alignment, allowing the abolition of several congested level crossings and the provision of double track.

From Alpnachstad station, the Brünig line follows the Sarner Aa and Lake Sarnen to Giswil.

Beyond the pass, the line descends its third and final rack section, down the steep side of the valley of the Aare, to Meiringen.

[1][3] From Meiringen to Brienz the line runs close to the Aare, in that river's valley.

Finally the line crosses the Aare on a high bridge, so built in order to allow lake shipping to reach Interlaken.

It then passes over the standard gauge access to the BLS AG works at Bönigen, before descending into its terminus at Interlaken Ost station, which is shared with the BLS AG and Berner Oberland Railway (BOB).

There is a physical connection with the BOB, which is also metre gauge, but again the lines are electrically incompatible and no through trains operate.

[1] The line is built to metre gauge (3 ft 3+3⁄8 in gauge), and operates on the rack and adhesion principle, using sections of Riggenbach rack to overcome the steeper gradients encounted on the approaches to the Brünig pass, but with most of the line operated by normal adhesion methods.

[1] The steam locomotives G 3/4 and HG 3/3 can operate on the rack sections only in the forward direction, hence turntables are needed.