http://railmap.railspot.com/maps/USA/CA/LosAngeles.html
You can zoom in and find what you need
This is what I wanted last week while I was in LA. UP had more parked trains than I could count extending from Colton in all directions. Can’t tell the yard from the main line of the UP without a map.
It’s a good overview… It’s probably about time to take out the old Valley Branch, and Pasadena line though. Unless they’re including light rail and express bus lanes. 
Dave
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http://eje.railfan.net/dpdp/
It’s not very acurate. It shows lines that were abandoned years ago and is missing yards like LATC, Gemco and others.
This link was posted a while back on another board. Main lines are accurately drawn, but as mentioned by Chad, Dave and others, there is some missing info; also there are some typos and misplaced names. (La Jolla is near San Diego; nowhere near Fullerton).
The ex-PE line that runs through Rancho Cucamonga to Rialto is gone (Rialto to Bench, on the Palmdale cut-off, is still there.