I'm unsure of exact train counts, but in the past decade or so after Donner Pass was cleared for double stacks, there hasn't been much. A good guess would be around 10 a day west of Keddie, but I'm not sure. If you want lots of trains, go to Donner, though (I think) the Feather River is more scenic. Here are the best spotting locations, from east to west:
One place you are sure to see lots of trains is the Western Pacific Railroad Museum in Portola, at the start of the grade. It is well worth several hours, and there are drive-a-train programs that you may want to research.
Continuing west, the line enters a canyon visible from Portola-McLears Road, but not CA 70. The Clio Trestle near Clio is impressive and may be worth a stop.
Further down CA 70 is Williams Loop, which is a fun spot.
CA 70 breaks away from the tracks slightly before returning near Keddie Wye. This is the most iconic spot on the whole line, and where the BNSF Inside Gateway line comes in. You'll see more trains west of here. Trees have grown up, but the spot is still a wonderful place and is a definite don't-miss.
Further down the 70 are the Honeymoon Tunnels adjacent to Rock Creek Reservoir.
The final famous spot is the overlapping bridges at Pulga. Another don't-miss.
Finally, due to a new dam and line relocation, CA 70 and the line share a bridge near James that is an interesting shot, however it requires standing by the roadside and extreme care to avoid being hit, so I can't recommend it. There are giant sweeping curves just west that are just as fun but safer.
Your best bet is probably to find a train and then chase it through the canyon. The trains move slow enough that this is generally safe and easy.
Some areas are relatively remote, so pay attention, make sure you have a way to contact the outside world as phone service is unreliable, and keep plenty of extra supplies. And, have a lot of fun!