I do have the AC-12 model, but it seemed a bit oversized on Rollins pass. Also, could somebody like some more information about this "Overnight", it sounds interesting!
The SP "Overnight" service was in essence the SP's second attempt at a dedicated high speed LCL service (including the SSW Blue Streak merchandise). Starting in the late 30's, the SP painted up AAR boxcars in a predominantly black and gold colored scheme to set them apart from other boxcars in their fleet. These cars would be assigned to a pool of LCL boxcars assigned to a number of trains between Los Angeles and San Francisco (with small segments branching off to the Pacific Northwest and Salt Lake City, although these would usually take the form of blocks of LCL cars cut into general hotshot freights). The GS-4s were only regularly assigned to the main freight runs, named the "Coast Merchandise," but there were other trains assigned in the time slot as well. Generally the only trains that would be solid SP "Overnight" boxcars would be the "Coast Merchandise," but a secondary service (generally handling overflow traffic from the first train), called the "Zipper" would (very occasionally I might add) host a GS class locomotive. The "Zipper" would be made up of mostly "Overnight" boxcars, but the difference is that this train ran on a slightly slower schedule and carried other loads aside from LCL traffic. Postwar, some of the rolling stock was painted in a silver scheme, but the vast majority of the fleet remained painted black until the service was eventually discontinued in the late 50's with the decline of LCL traffic.
One thing to note, before y'all get any ideas, is that the Overnight service only ran on the Coast Line. In the event of a reroute, the schedule would not be able to be kept effectively, and the cars would be cut into other freights heading via Tehachapi or other routings instead of operating as a solid block.
Perhaps the most interesting part about these trains was that they were spread in with the other SP overnight trains on the Coast Line. The lineup from the late 30's would most likely be roughly about this (mind you I don't know departure times, I can only go off of accounts from trainmen and various articles that I've read. I'm sure that a timetable detailing the exact times can be acquired, but for now I'm just gonna give the order of train movements).
Owl
Zipper
Lark
Overnight
Starlight
That just accounts for the main passenger and freight runs, not extras, general merchandise freights, and secondary passenger trains (of which there were plenty).
To end this TL;DR off, here are some pictures of different Overnight boxcars that the SP operated. The wood boxcars were used from the inception of the service in 1935 up until the late 40's, the black AAR boxcars were delivered in 1939, and the silver scheme came about in late in the service's life, around 1955.:
EDIT: One other thing to add: these cars were rarely, if ever taken off of SP rails. There isn't any proof of these cars ever having been operated outside of California and Nevada (and occasionally Oregon and Seattle, but I'm just inferring this as there's no evidence to support that), and even then they were pretty much limited to the mainlines (the Coast Line, Tehachapi, etc). So the possibility of these cars operating in trains not operated by SP is pretty much none.