The reason for this is that the walls of the canyon are so steep that there are multiple places where rocks can dislodge and this presents a serious threat to the train. This railway was constructed over 100 years and was completed in 1961. The numerous tunnels and bridges were constructed with lots of movement of rock and rock faces.
This rock threat is especially acute in the heavy rainy season. Once in the month of October, we were stuck on the Primero train for 18 hours in a downpour. There were rocks in front of us and behind us. Crews worked all night to get the tracks cleared and we moved on east at sunrise.