Hello there Owlroost,
I live just east of Roseville, so I do most of my railfanning in and around Donner Pass. A few things I can recommend:
Get a good map. Trains Magazine earlier this year had a"map of the month" called "Donner Pass, then and now". This map is so good I keep a copy of this map in my car. One nice thing is that it has the I-80 freeway exit numbers so you'll know where you are. If you can interpret topographic maps and have a smart phone it is good to use a App like "US Topo Maps". I use these topo maps when checking out possible photo locations, like hilltops in the back of Cold Stream Canyon for example.
You don't have to go "offroad" with your rental car, but you will want to get away from I-80 to see the railroad away from the noise of the freeway. Some of my favorite places include the area near Casa Loma (exit 146) and Cold Stream Canyon (Truckee, exit 184). Make time to go on the historic Lincoln Highway between Soda Springs and Donner Lake. The old number one track between Norden and Eder Canyon is no more, but you can still see the historic summit tunnel and the snowsheds that run below the summit of Donner Pass.
Don't forget that the summit of Donner Pass is over 7,000 feet above sea level. If you are planning on going there in October and staying in Truckee be prepared for mornings below freezing. Make sure you have the right gear and clothing, even if you don't plan to go to far from the road. Also, there are many areas where there is no cell phone coverage, such as Casa Loma and the back of Coldstream Canyon. Keep this in mind when you are traveling over the mountain.
Some of my favorite places to photograph, from Roseville to Reno: The SP station at Roseville-plenty of trains, Colfax, Gold Run and Alta, Casa Loma and Rocky Point (on the edge of the American River Canyon), Emigrant Pass and Yuba Gap, Soda Springs and Norden. You can park near the Donner Monument and hike back into Cold Stream Canyon (near Truckee). Here the track heads south to the back end of the canyon, then reverses itself to climb to Andover, and overlook Donner Lake. Below Truckee you can get off I-80 and check out Boca, Farad and Verdi, before dropping into Reno.
I'm not sure what the frequency of trains are over Donner Pass. I've spent Saturdays up on "the Hill" and seen as many as 4 trains in 1 hour, to other times when I saw just one train all day. One reliable thing is that Amtrak trains run each way over the mountain daily.
If you want to get some ideas of what to photograph in the Donner Pass area, you can see some of my photos in a portfolio called "Steel Rails over Donner Pass". Here is the link: http://www.blurb.co.uk/b/4715599-steel-rails-over-donner-pass
Well, that's about it. Happy Railfanning on the Hill !!!