The tunnel was widely disliked by railway personnel during the era of steam operation due to the danger of gas buildup.

The crew of passenger train #3616, which left Wattwil at 11:05 am on October 4, 1926, and arrived in Kaltbrunn at 11:20 am, did not notice any significant gas buildup in the tunnel.

[3] The train, led by a B 3/4 steam locomotive, had 6 Swiss Federal Railways personnel aboard and a trailing load of 252 tonnes (278 short tons).

[4] As the train continued through the tunnel, its speed gradually decreased due to insufficient steam generation.

After 6+1⁄2 minutes or 1 kilometre (0.62 mi) of travel, the train got stuck on the incline because it was using low-quality fuel briquettes.

Due to poor ventilation in the tunnel, there wasn't enough oxygen for the steam locomotive's fire, resulting in incomplete combustion.

[2] According to the Swiss Federal Office of Meteorology and Climatology, there was an exceptionally strong anticyclone (high pressure area) on the north side of the alps on the day of the incident.