There are a number of locations to shoot down on the Alameda Corridor, where it enters upper Alameda from some of the new bridges. It is harder to get good shots in and along the trench.

East of the former SP LA Shops intermodal complex along Valley Blvd, and Mission Road, in El Sereno (also home of the famous Pasadena Model RR Club) you can set up and shoot shots of trains arriving on the Sunset Main from Colton. Trains out of the East LA Yard on the original UP, are found from Montebello to Pomona (where both lines come back together, with many good spots for photos.,d

The Harbor, especially Terminal Island, can be tricky due to way more Port Security, many closed areas, and a lack of legal parking in many locations. But, there are still a number of good vantage points, and some on the new bridges where you can get some pretty good views of rail operations, around San Pedro, Wilmington, and on Pier J in Long Beach (Queen Mary).... The famous Henry Ford bridge, where trains come onto and off of Terminal Island is a classic location, but with difficult parking.

If you are not from the LA area, you should try to familiarize yourself with some of these areas on google earth first, and remember greater "LA" is actually sometimes "bigger" than people expect, in regard to both distances and freeway congested travel times...