BNSF 9100 (SD70ACe) assists Amtrak's California Zephyr as it runs through the yard in downtown Denver, Colorado.

UP 737 (ex-Missouri Pacific GP38-2) waits in the North Yard in Denver, Colorado for the call to action to sort freight cars.

A variety of UP power sit in the North Yard in Denver, Colorado.

The Union Pacific's light motor of choice to work the North Yard in Denver, Colorado include GP38, GP39 and SD40N models. Here we see UP 1826 getting ready to pull a string of cars out for sortin... (more)

UP 737 (GP38-2) sits ready to move some oil cans in the North Yard in Denver, Colorado.

Former AT&SF Bluebonnet #2854 (GP39-2) teams up with former Burlington Northern #1448 (SD60M) as they work sorting cars in downtown Denver, Colorado.

UP 1019 (GP60) heads into the North Yard where the pair will sort cars in the north end of the yard in Denver, Colorado.

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

BNSF 2718 (GP39-2) and a bluebonnet team sort freight cars in the yard in downtown Denver, Colorado.

BNSF 7080 (ES44C4) leads a freight through the North Yard in Denver, Colorado.

Fresh snow, a breaking dawn, and sub-freezing temperatures greet the California Zephyr as it passes a BNSF freight in the Denver Yard.

A fresh layer of snow blankets the Union Pacific's North Yard in Denver, Colorado.

The crew of a pair of yard switchers catch a ride back to the head of the North Yard in Denver, Colorado.

UP 1640 (SD40N) joins other light power as they sit in the North Yard in Denver, Colorado waiting for assignments.

The blue hour begins to give way to the pink color of sunrise as BNSF 2712 (GP39-3) works a cut of cars in the yard in a snowy downtown Denver, Colorado.