The K-Line E-9s are 17.5 inches each, too long for the 15-inch passenger cars. K-line made F-series diesels for most lines, but for Southern Pacific, I only recall seeing them in the Black Widow paint scheme, not the Daylight scheme.
I agree with the other guys above who recommended the K-line GS-4, which is larger than semi-scale but not full 1:48, perhaps closer to 1:58 like their slightly compressed Big Boy and Alleghaney, to go with these cars. It came in a conventional version and a loaded one with the following features:
K3688-4449CC The most distinctive features of the GS-4 are replicated in this model version, including the curved steps leading up to an enclosed all-weather cab and operating MARS light installed above the headlight on the aluminum smokebox door. The K-LINE engine also features an opening cab roof hatch and pilot truck detail unlike any other GS-4 model. Plus it has Cruise Control, TrainMaster Command Control, Lionel RailSounds and Electrocouplers.
I found one a few months ago, reasonably priced, and have seen a few more for sale since. The only catch is that K-Line cruise control is notoriously bad, so plan on disabling it.
The conventional model # is K3688-4449W. See the K-Line Legacy site for more details on both versions.
Best of luck!
David