I would vote with the others for Fullerton Station. It has lots of traffic. And there are a good many restaurants, grills, and bars within 3 blocks. Oh, yes, and a parking lot. That makes a big difference.
Cajon Pass is a nightmare on the Interstate. The Interstate is always busy, and extremely so in the morning and evening with commuters. The old highway has a scenic pull-off at Blue Cut. There are sometimes men looking for something besides trains lurking there, but others go there to watch trains and apparently are not bothered. Summit is a well-known place to watch trains, overlooking the big curve in the cut between Summit and Walker, but that road has been improved and I can't say if you can still park at the curve. If you are used to high speed heavy highway traffic, Cajon Pass could work. You'll need to carry something to eat and drink, as it's a drive to the nearest location to buy any. And there's no shade.