Much of doing model railroading is research, you get a lot of book and internet time. To help launch you into the past here's a link to an SP society page.
By the late 1930's the Pacific style 4-6-2 locomotives were falling back to either pretty flat country or secondary trains because they didn't make enough horsepower to maintain schedules in mountains and hilly country especially with the larger consists that were becoming common before and during WW II.
My childhood memories of taking the SP commuter trains between San Francisco and San Jose is of pretty somber trains but this was late WW II and immediately after so fancy orange paint schemes were replaced with black and gray.
http://espee.railfan.net/passenger.html
Bogie
