To add to Jim B's comments, if you want to do some chasing, both railroads south of Stockton offer possibilities. UP runs parallel to Highway 99 from Manteca all the way down to Bakersfield. A scanner and Altamont timetable are helpful, or you could just drive down the freeway until you see something.
BNSF is parallelled by county roads (usually called Santa Fe Avenue/Drive/Road) from Escalon to Riverbank, and then from Empire all the way to Merced and beyond. Traffic on both railroads varies by day of the week and time of day, so how much you might see is a matter of chance.
The Modesto & Empire Traction Co. serves a large industrial area along Highway 132 (Yosemite Blvd.) between Modesto and Empire with a fleet of new gensets. They run several jobs on weekdays, but little or nothing on weekends.
Altamont Pass (SE of San Francisco, between Livermore and Tracy) is photogenic but doesn't have much traffic--three Altamont Commuter Express passenger trains each way between Stockton and San Jose on weekdays, and maybe three UP freights each way a day.
The South Bay (San Jose area) has tons of passenger trains (Amtrak/Caltrain/ACE) but very little freight traffic. The urban sprawl and congested roads make railfanning there difficult.