Idrija has always strived for a railway connection to the world but only got it during the First World War – and even that for a very short time. The basic purpose of this railway was supplying the Isonzo Front, which is why it went – more or less by coincidence – through Idrija and to Dolenja Trebuša.
After the 6th Battle of Isonzo with Gorizia falling in August 1916 and the supply of Austro-Hungarian army via the Bohinj Railway was cut, the battlefield supply went mostly from Logatec under the Banjšice Plateau and along the Isonzo (a part of the battle line between Gorizia and Tolmin). To enable it, a prefabricated railway called feldban (from German Feldbahn) was rapidly laid on the existing roads from the Logatec railway station to Godovič, and then further on to Črni Vrh, through Zadlog and the wide forests of the Trnovo Plateau towards Lokve.
Between 1 and 20 September 1916, a similar narrow-gauge railway leading towards Idrija was laid, too. Here, several kilometres of the tracks had to be made anew as, in places, the existing “French Road” was too steep and had too many hairpin turns. Just before the Battle of Caporetto at the end of October 1917, this railway fork was extended to Dolenja Trebuša, where the cargo was transloaded onto cableways.
It was quickly clear that narrow-gauge railways would not sufficiently supply this part of the front and a standard track running from Logatec to Črni Vrh and onwards to Col and Ajdovščina was immediately began being traced. Ground works on the terrain began in early 1917, and by October, the majority of works in the Logatec–Kalce section were done (the Naklo and Kalce tunnels, the military railway station west to the Napoleon Avenue in Dolnji Logatec, embankments, and cuttings). The section towards Godovič was only partly finished (cuttings, embankments); both tunnels (160 and 380 m in length) were broken through and partly completed. This extensive work was finished in such a short period of time on account of war prisoners, mostly Russians. There were supposedly around 20,000 of them, with the same number of horses working, too. The prisoners were installed in war barracks in various places – wherever the needs occurred.
The Idrija section of the railway forked out at the Cestnik Turn south of Godovič, ran on the French Road for a little over 3 kilometres, and then ran over new terrain down the steep slopes and rock walls above the Zala Valley. On top of the Ključe Road above the Wild Lake, the railway crossed the French Road over a bridge and turned towards the Bela or Idrijca River Valley. To reach the turning point where the trains changed direction above the Strug Valley, a 25-m tunnel had to be drilled into bedrock and immediately after it, a gallery was built in the same length. Right before the Battle of Caporetto, a semi-circular wooden bridge was constructed at the turning point which probably largely improved the openness of the track. From there on, the tracks drop for another 50 metres in altitude, reaching the road to Idrijska Bela around a kilometre before the Wild Lake.
The entire length of the feldban with both forks, i.e. the one leading through Črni Vrh and the Trnovo Forest towards Lokve, and the one through Idrija to Dolenja Trebuša, was exactly 100 km. On this narrow-gauge railway, the carts were first driven by horses and later by small locomotives.