A trio of SD40's are at the ready in the Union Pacific's North Yard in Denver, Colorado.

UP 1957 (SD40N) leans on the tack at the North Yard in Denver, Colorado.

UP 1640 (SD40N) and UP 1344 (GP40-2) wait in the North Yard in Denver, Colorado for orders.

UP 1957 (SD40N) sorts cars in the Union Pacific's North Yard in Denver, Colorado.

The operations building in the Union Pacific's North Yard in Denver, Colorado has a commanding view of the massive yard.

UP 1640 (SD40N) moves through the BNSF's Denver Yard after dropping off a cut of cars.

The first light of the morning finds UP 1957 (SD40N) positioning to shuffle cars at the North Yard in Denver, Colorado.

UP 1957 (SD40N) runs with open doors in the Union Pacific's North Yard in Denver, Colorado.

UP 1826 SD40N get ready to sort a string of cuts in the bathroom.

The doors are open on a hot day as UP 1957 (SD40N) heads for the barn after a day of sorting cars.

The remote operator of UP 1957 (SD40N) catches a ride back to the barn after a day of sorting cars in the North Yard.

UP 1817 (SD40N) heads deeper into the North Yard in Denver, Colorado to sort train cars.

UP 1817 (SD40N) gets repositioned in the North Yard in Denver, Colorado.

UP 1957 (SD40N) moves south in the North Yard as the Amtrak's Winter Park Express Ski Train heads north.

Snow clouds are moving in as UP 1957 (SD40N) works the North Yard in the evening sun.