I think the last time it was used was following the terrible truck-Amtrak collision east of Reno in June, 2011 when it was re-routed for a few days. Perhaps others are aware of this detour on other occasions.
I thought I remembered something like that... but didn't want to assert such with only a very vague recollection...
Any guess on the time penalty of going that route? (beyond that: what is the actual routing from Reno to SAC? ... and the SAC end rejoin the over-the-top route before SAC, or at?)
The Feather River route (former Western Pacific) is longer than the Donner route. There is a time penalty for taking it, besides the delays related to having a UP pilot. Although it was touted by the WP as a scenic selling point for the original California Zephyr, arguably the Donner route (former Southern Pacific) is the more scenic of the two. The Feather River route is scenic, but in my opinion, the Donner route is more scenic.
Heading westbound, the Feather River route departs from the normal CZ route at Winnemucca NV. It heads due west (as the CZ heads southwest) to Portola CA. It then follows the Feather River (CA Route 70) though the northern Sierras to Oroville, then south to Yuba City where today it joins the route used by the Coast Starlight on to Sacramento.