The origins of the Appenzeller Bahnen company lies in a number of formerly independent companies and railway lines:[4] The Appenzellerbahn and Santisbahn merged in 1947, retaining the Appenzellerbahn (AB) identity.

From 2016 to 2018, the Appenzellerbahnen undertook a large construction project to connect the Appenzell-St. Gallen and St. Gallen-Trogen lines.

The three points of incompatibility were electrification (the St. Gallen-Trogen line was 1000 V DC with a brief stretch at 600 V DC shared with the St. Gallen trolleybus system), different (but physically adjacent) termini in St. Gallen, and maximal grades (the rack railway approach to St. Gallen from Appenzell was too steep for adhesion-based St. Gallen-Trogen rolling stock).

So the new project re-electrified the St. Gallen-Trogen line at 1500 V DC and constructed a new tunnel through St. Gallen.

The old alignment through St. Gallen closed in April 2018, and the system began through-running in October of the same year.