Good morning, Frogpipe,
I've been an Espee modeller for well over 50 years, but my emphasis has always been the SF Bay Area and the Coast Line and Western Divisions. There are a few things I know of the Siskiyou line, though.
The Natron Cutoff was built to bypass the Siskiyou line's curves and grades and offer an alternative route to Eugene in the mid 1920s. Ever since the Siskiyou has been very much a secondary route. That means that in the 1930s there would be freight and local passenger service. I do not believe there ever was commute service anywhere along the line, even when it was the main connection with Eugene. In that Natron cut 25 miles off of the trip to Eugene, and had easier grades and more relaxed curves - one curve on the Siskiyou main was 15 degrees, or radius 380 feet! - bridge freight and most passenger service would have been removed from the Siskiyou line, though some passenger service remained until the mid-50s. Industries primarily feature lumber and orchard fruits.
Main line motive power in the '30s would include F4/5 Decapods (2-10-2s on the Espee; wouldn't do to run "Santa Fes" on the Espee), and older cab forwards, including some AM-2s (available from Paulz Trainz) I believe, came into use there as well, though I'm not sure if this was in the 30s.
There is a book available titled "The Siskiyou Line - Documentary: Adventure in Railroading" that is supposed to be very good, though I confess to not having seen it and don't know if it focuses on your era.
I think you are going to have to do your impression of Indiana Jones to get some really good info about this line, but it should be fun for you and I'm sure we will all enjoy following your progress. You might want to start at the SP Modellers Society forum here
http://www.southernpacificmodelerssociety.org/index.php They are V-scale friendly and have been extremely helpful to me.
Bernie