UP 1837 (SD40N) notches up as it pulls a cut of cars out for sorting in the North Yard in Denver, Colorado.

UP 1837 (SD40N) runs solo into the North Yard in Denver, Colorado.

In this wide-angle view of the Union Pacific's North Yard in Denver, Colorado, there are a variety of light and heavy power occupying numerous tracks at the large facility.

Two crewmen ride UP 1837 (SD40N) into the North Yard where they will sort a cut of cars in the north end.

UP 1837 (SD40N) runs west on the Belt Line towards Denver, Colorado.

UP 1837 (SD40N) runs west towards the North Yard in Denver, Colorado.

UP 1837 (SD40N) runs on an elevated track past a power plant in Denver, Colorado.

UP 1837 (SD40N) runs over an old Rio Grande bridge in northeast Denver, Colorado.

A group of Union Pacific light power sits in the North Yard in Denver, Colorado.

UP 1837 (SD40-2) works sorting cars in the North Yard in Denver, Colorado.

UP 1837 (SD40-2) moves a flat car and an oil can deeper into the North Yard in Denver, Colorado.

UP 1837 (SD40-2) shuffles cars in the North Yard in Denver, Colorado.

UP 1837 (SD40-2) works the North Yard in Denver, Colorado.

A group of SD40's hold station in the Union Pacific's North Yard in Denver, Colorado.

UP 1837 (SD40N) sorts cars into the ladders at the Union Pacific's North Yard in Denver, Colorado.