In Union Pacific parlance, a "Z-train" historically means a priority train consisting of time-sensitive commodities, usually TOFC (trailer on flatcar) or single/double-stack (intermodal) well cars. Here, well cars carry single-stack containers (click on photo to enlarge).
This Z-train is rather unusual insofar as it is fronted by UP 4579, an EMD SD-70M with 4,000 hp and followed by two other EMD SD-70s. A pure EMD consist is extremely rare indeed.
Caught at milepost 148 on the number one (westbound) track, you can see it's about to hit the detector ahead. Note the flaps spanning the width of the track. This detector can sense dragging equipment, hot boxes and locked wheels and axles.
Freight speed is generally limited on the number one (downhill) track to 25 mph; it is easy to hear traction motors howling in dynamic braking. Engineers run about 10 pounds of air in the brake pipes downhill.
A brief note about single- and double-stack containers over Donner Pass: UP can only run single-stack containers on the Number One and Number Two tracks, as its older Transcontinental Railroad tunnels are insufficiently tall. UP would have dug down into these tunnels in order to accommodate double-stack, but met resistance by enviros who objected to the alleged "ruination" of these historical tunnels.
Therefore, east/west northern California UP double-stack traffic must run over the Feather River (formerly Western Pacific) route instead of Donner -- but this adds another 70 miles to the journey. Therefore UP would rather run double-stack over Donner and save mileage but cannot yet do so.
That is why you will only see single-stack intermodal Z-train traffic over Donner Pass on the Roseville Subdivision.
MP154
The camera had much difficulty finding its focus in the heavy snow, and the train was having its own set of difficulties as well. Sounding to be in Run 8, the locomotives were working for all available traction, sanders on overtime.