The completion of the railway was not without its problems. In a push to finish work quickly, the descent from the Kicking Horse Pass to the river valley was built on a slope with a steep grade of 4.5%, more than twice the maximum allowed according the CPR directives. Numerous runaway lines failed to prevent numerous derailments on the Big Hill, leading to many proposals to rework the line. Construction on the Spiral Tunnels began in earnest in 1907, driving the trains through two loops deep inside the mountains and reducing the slope of the descent to 2.2%. The Spiral Tunnels are in use to this day, and easily accessible viewpoints are a popular destination for visitors to Yoho National Park

In this image the trains go into the mountain at the lower right tunnel and emerge from the mountain at the upper left. If one is lucky, which I wasn't, one can see the train emerging from the upper tunnel and the end of the train still going into the lower tunnel. This is the upper spiral tunnel.