The railway museums that I know of, and am learning about, in Southern California... a lot more of them then I would have guessed

http://www.oerm.org/pages/visinfo.html for directions and the rest of the 2012 calendar of events is at http://www.oerm.org/pages/calendar.html
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The Poway railway museum is at http://www.powaymidlandrr.org/default.htm and I posted about it a couple years ago: http://justacarguy.blogspot.com/2007/06/any-day-around-trains-is-good-day.html

get the links and check out their websites at http://www.daytrippen.com/pacific-southwest-railway-museum.html