UP 1357 (GP40-2) works the North Yard in Denver, Colorado.

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.

The California Zephyr leaves Denver Union Station and passes under the Park Avenue Viaduct on its way to the Moffat Tunnel sub and the Colroado Rocky Mountains.

An electrician carefully checks a circuit board from Union Pacific GE ES44AC No. 7682 on Track 5 inside the Diesel House at Union Pacific’s Burnham Shops on December 16, 2015. These former Rio G... (more)

Union Pacific UPY 577 has maintenance work done on Track 2 in the Diesel House at the railroad’s former Rio Grande Burnham Shops in Denver, Colorado, on December 8, 2015.

A Union Pacific Officer's Special train sits in a snowstorm in the North Yard in Denver, waiting for passengers to arrive.

The snow is falling fast as UP 4404 (SD70M) waits for passengers to load up an officer's Special passenger train, which will run up the Moffat Tunnel Sub and through the Continental Divide.

BNSF 9322 (SD70ACe) joins fellow pumpkins and a Warbonnet in the BNSF engine servicing yard in Denver, Colorado.

Falling snow covers both the California Zephyr as it leaves the North Yard and the Union Pacific Officer Special passenger train that will soon follow the Zephyr up the Moffat Tunnel Sub.

The Union Pacific's North Yard in Denver, Colorado is the staging point for a number of Maintenance-of-Way equipment, including a few Plasser American Tamper machines.

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

BNSF Warbonnet 712 (Dash9) is waiting for an assigned train after getting service in Denver, Colorado.