It's just after sunset with temperatures close to zero as Amtrak train no. 6, the California Zephyr, wastes no time descending the grade at Coal Creek Canyon during the cold blue hour.

With plenty of alpenglow on the Front Range mountains very early on the morning of January 22, 2007, an eastbound Union Pacific coal train drops downgrade just west of Leyden, Colorado.

With the temperature close to zero, Amtrak train no. 6, the California Zephyr descends the grade at Big Ten Curve with the Denver Metro area in the background.

It's just after sunset as Amtrak #6 eases down the grade around Big Ten Curve at dusk with the Denver Metro Area in the background.

A westbound Union Pacific coal train catches the last sunlight of the day as it exits Byers Canyon on a sharp curve between Sulphur and Flat, Colorado, on June 3, 2013.

Union Pacific GE AC4400CW-CTE No. 5990 leads an eastbound coal train out of Tunnel 1 and curves downgrade toward Coal Creek east of Plainview, Colorado, on April 30, 2014.

Amtrak train no. 5, the California Zephyr winds around curves above Big Ten and heads into tunnel 1 on the climb out of Denver up the Front Range of the Rockies. At this time Amtrak was attempting... (more)

An SD70ACe and AC4400CW lead a Union Pacific loaded coal train bound for Memphis, TN just west of Leyden near Arvada, CO as it continues to get colder and the rain continues to fall.

An SD70M, AC6044CW, and SD40N lead the North Yard, CO to Grand Junction, CO manifest through Coal Creek Canyon west of Denver as the rain continues to fall.

Rain, fog, and wheel slip. That pretty much sums up of what's happening here. An SD70M, AC6044CW, and SD40N lead the North Yard - Grand Jct manifest as they climb the grade at Big Ten Curve.

E SLPRWM strings out uniform coal hoppers in front of one of the Mooney Hills on the approach to Beshoar siding in extreme southern Colorado.

Union Pacific’s MRONY freight drops off of the mesa at on Big 10 Curve west of Rocky, Colorado, on June 28, 2014. Today’s train is fairly short, with the lone DPU locomotive visible on the opp... (more)

It’s 8:25 p.m. on the evening of August 3, 2014. A low rumble can be heard along the Colorado River under deep blue colors of a fading day in the valley surrounding Radium, Colorado. Another coa... (more)

A CitiRail leased ES44AC no. 1331 and two BNSF C44-9W's ease the Provo, UT to Denver, CO manifest down the grade from Big Ten Curve. The Denver Skyline and suburbs can be seen in the background.

A Union Pacific inspection train approaches Big 10 Curve west of Rocky, Colorado, just after dawn on July 26, 2014. EMD SD70ACe No. 8734 leads power car UPP 207, sleeper “Portola” UPP 1610 and... (more)