Truckee, CA on the east side of Donner Pass can be a pretty good sight for watching trains on the ex-SP Donner Pass line. Traffic at times can be a little 'iffy' , but UP is starting to put more "Overland" trains on Donner these days, plus the twice a day "California Zephyr" between Chicago and Oakland, which--if on time--goes through westbound around noon and eastbound around mid-afternoon.
The last time I was there, the afternoon seemed to be the best time for train-watching. UP ran about six trains through, both east and westbound in the space of a little over two hours. The old SP classic-style Truckee passenger station (now AMTRAK) is a perfect place to watch them and is hassle-free. And the surrounding scenery is spectacular. And Truckee itself, is a very neat mountain town with some good restaurants.
Best time is spring and summer--the winters can get a little ferocious up there, unless you're used to snow--and a LOT of it! 
Tom