Spiral Tunnels, British Columbia, Canada
The Big Hill on the Canadian Pacific Railway main line in British Columbia, Canada, was the most difficult piece of railway track on the Canadian Pacific Railway's route. It was situated in the rugged Canadian Rockies west of the Continental Divide and Kicking Horse Pass. Even though the Big Hill was replaced by the Spiral Tunnels in 1909, the area has long been a challenge to the operation of trains and remains so to this day.
The essential problem was that the railway had to ascend 1,070 feet (330 m) in the space of 10 miles (16 km) from Field at 4,267 feet (1,301 m) climbing to the top of the Continental Divide at 5,340 feet (1,630 m). The narrow valleys and high mountains limited the space where the railway could stretch out and limit the grade (hence the later decisions to bore extra mileage under the mountains and lower the grades)