Construction of the Spiral Tunnels commenced in 1906 with completion in 1909.
The tunnels were constructed to replace the 4.5 % grade of the Big Hill with a
gentler grade of 2.2 %. Alternative solutions to grade reduction were
considered, namely following the east and west slopes of the Yoho River to the
north or maintaining a longer grade on the south side of the Kicking Horse River
passing Field at a higher elevation. But it was concluded that both of these
solution posed too high a risk for avalanche and mudslides.
J. E. Schwitzer, one of the railway’s Assistant Chief Engineers, came up with the
idea of two Spiral Tunnels. After cresting the Kicking Horse Pass a westbound
train proceeds to the 3255-foot Upper Spiral Tunnel. Also known as Spiral Tunnel
#1 the train completes a 3/4 of a circle left-hand turn in Cathedral Mountain
emerging 56 feet lower. After descending to the northeast the train enters the
2922-foot Lower Spiral Tunnel (Spiral Tunnel #2) were it completes a two-thirds
of a circle right-hand turn. It emerges 50 feet lower again heading west towards
Field, British Columbia.
As many as 30 trains a day pass through the Spiral Tunnels. The lowering of the
grade increase the efficiency of eastbound locomotives, while improving safety
for westbound trains as they descended the hill. Cost savings were realized by
allowing the removal of many helper locomotives from dedicated service at
Field and Laggan (now Lake Louise).