The Denver & Rio Grande built the San Juan extension into SE Colorado mining regions in 1879 -81, many times requiring engineering and grades that pushed the limits of traction locomotive power to get trains over 10,000+ foot passes. The west side of Cumbres pass hits 4% gradient, requiring eastbound movements to be broken into "turns", .... smaller car groups that could be dragged upgrade to a reassembly yard, where the entire train would be put back together and brought down the other side. Here we see "Mudhen" 463 switching freight on a rather cold but typical winter day in the high country.