M636C's reply is fairly comprehensive, but I'll add my $0.02.
When I was in LA for a short holiday in 2012, I rented a car from Hertz for 3 days. It was prepaid by my travel agent, so the pickup was simply the usual rental rental procedure - confirmation of license and credit card details (again, Aussie license is ok for a tourist. I think it only becomes an issue for an extended stay).
I picked the car up in Santa Monica, but made my way out of LA almost immediately. This was my first time driving on the right, so I think starting in LA gave me an advantage in that there was *no* temptation to drift to the left hand side. Also, I hired in the mid morning, so the freeways were not parking lots
My first day was non-train spotting (I couldn't pass up a look at the SR71 museum in Palmdale), but I spent the night at Cahon Junction, where there is a Best Western Hotel that's quite reasonable.
Just south of the junction is a spot off the I-15, where Cajon Boulevard goes right beside the railway line, and I spent a little time there the following morning, catching a couple of BNSF and UP services.
As far as Cajon Pass went, tales of BNSF & UP police kept me on the straight and narrow
. I spent an hour or two beside the road there. Further encouragement was in the fact that I hired a compact Nissan, which probably wouldn't have gone too well on the dirt roads beside the line. And the fact that most of the tracks are in a national park (or similar) which requires permits sealed the "stay on the main road" deal for me.
Map of Cajon Junction and Boulevard spotting place
I travelled to the Western American Railroad Museum at Barstow, for the main reason that one of their exhibits is an FP45 in Warbonnet livery. It's not a huge museum, but it's definitely interesting. That was more or less all the trainspotting part of my journey. http://www.barstowrailmuseum.org/index.html
Couple of car related things:
The highway to Barstow is about 100miles, which by Aussie standards isn't really *that* far. The speed limit is either 65 or 70, I can't remember which. It's 4 lanes for most of the way: The far left is for two or more occupants only, the mid left is for the uber confident/crazy people for whom the speed limit is a suggestion (including at least two LA County Sheriff and Highway Patrol cars that passed me by). The mid right is for slightly nervous newbie folks such as myself, who are content to stay about 5mph over the limit. You should probably stay out of the far right, as this is one where slow semi's travel in, and also turns into an "exit only" lane without warning.
I hired an automatic, as I figured I would getting into enough trouble with indicators and rear vision mirrors, without having to worry about what gear i'm in. And I would still be over there without my trusty GPS.
Also, the contract allowed me to keep the car an addtional day or two without having to notify the company. I don't hire a lot of cars, so I don't know if this is a common thing or not. It was certainly handy in my case.
I also found this site - written by an expat Aussie - to be quite helpful. I had about 3 horns honked in my direction during my stay, without it, it probably would be more right turn on red.http://www.californiadriving.com/