Well, I don't know of any trackside motels in the LA area (at least in decent, i.e. safe, locations) with good views of the tracks.However, depending on where you are in LA, how far you want to go, and what you want to see, there are quite a few decent locations to watch trains:
Metrolink stations are probably tops on the list since they are open to the public and easy to find (just consult a map). Most of these locations have both freight and passenger traffic, although traffic volume varies significantly depending on what line you choose.
LA Union Station is always nice if you go for lots of passenger activity.
There are some BNSF and UP yards in LA (like Hobart or Dolores) that have sections that are viewable from public streets, although I wouldn't go anywhere near their entrances.
If you're willing to drive a bit and can find your way around, Orange County has some nice locations, particularly Fullerton station with lots of BNSF and Amtrak traffic.
And of course, I'll make a pitch for UP's Santa Ana Branch in Anaheim, which probably has the longest section of operating street running in Southern California (see posts on this board).
[This message has been edited by atsf3751 (edited 05-19-2003).]