Gallen–Trogen railway, also known as the Durchmesserlinie (diameter line), is a metre gauge interurban tramway in the Swiss cantons of Appenzell Innerrhoden and Ausserrhoden and the city of St. Gallen.

The difficult line with tight bends demanded new design solutions to enable the BCFeh 4/4 electric railcars to operate over both adhesion and rack sections.

[11] With a gradient of up to 7.6%, the Trogen Railway was the steepest narrow-gauge adhesion line in Switzerland prior to the opening of the Ruckhalde Tunnel.

In order to eliminate the uneconomical service pattern of the trains in the St. Gallen "side" station (Nebenbahnhof) needed to be connected to the Appenzell–St.

The required work was subdivided into five sub-projects, of which the St. Gallen station–Riethüsli section, including the proposed Ruckhalde Tunnel, which was approximately 700-metre (2,296 ft 7 in)-long, was the most complex and largest.

[13] Teufen station received a third platform track, so that besides the crossings of the regular interval services, trains can reverse there in the peak.

The siding to St. Gallen AB station was replaced by a crossover and the double track was extended from the Rathaus (town hall) to the St. Leonhard bridge.

In order to simplify operation, the seven signal boxes between Niederteufen and Trogen have been replaced by two new systems, which contain components of the SiGrid interlocking architecture, developed by Siemens.

The elimination of the technically complex and expensive to operate rack section in the Ruckhalde has made it possible to use the newly acquired Tango (class ABe 8/12) low-floor trains, which are more comfortable and quieter, but also more cost-effective.

The Ruckhalde Tunnel route eliminated the need for six level crossings, which has significantly increased traffic safety in the Riethüsli district.

Running next to the road, after passing over a watershed, the railway reaches the crossing station of Sammelplatz and, after about three kilometres of down grade, the entrance of the Ausserrhoden village of Gais.

Since 2018, the line has run through the Ruckhalde Tunnel on a decline of 8.0%, losing about 70 metres (229 ft 8 in) in altitude, to the level of the once important SBB freight yard in St. Gallen.

The tunnel replaced the last remaining rack section, which had impressive view of the western parts of the city, and the sharp Ruckhalde curve.

The tunnel also made an approximately 300 metres (984 ft 3 in)-long section of overhead line that had been shared with the St. Gallen Trolleybus superfluous.

[14] After passing under the St. Leonhard bridge the line reaches the so-called St. Gallen Nebenbahnhof ("side station"), where a connecting building provides covered access to the standard-gauge trains of the SBB, SOB and Thurbo.

Originally, the trains to Trogen ran in the city on the tracks of the St. Gallen Tramway, which was closed in 1957, but this rail infrastructure now belongs to the Appenzell Railway.

After taking over the tram infrastructure from the city of St. Gallen in 1959, the Trogen Railway was required to remove the second track in Bahnhofstrasse (between the station forecourt and the Schibener Gate—Schibenertor).

In this section, the line has the character of an interurban, with trains passing on the left at the crossings loops, as in usual on Swiss railways, unlike trams.

[22] The two minutes reduction in running time enabled by the Ruckhalde Tunnel required the extension of the Liebegg crossing loop to the Lustmühle halt.

[12] Since the opening of the Ruckhalde Tunnel, all traffic has been handled by eleven 2.40 metre-wide Tango articulated railcar (class ABe 8/12), which is determined by the structure gauge of the Trogen Railway.