It is completely legal to photograph the Los Angeles Metro System.
On October 31, 2009 while on my way home from the Hollywood and Highland area, I was unlawfully detained for 25 minutes by LASD Officers Richard Gylfie #2955 and Bayes #456 for taking two photographs of the turnstiles located at the Hollywood and Western Metro Station -- an act that is completely legal and occurred on public space.
As you can see in the video, Officer Richard Gylfie #2955 and Officer Bayes #456 took it upon themselves to ignore Metro policies in order to bully me, humiliate me, and detain me for conducting a perfectly legal activity in public. More important, by illegally detaining me and threatening to have my name added to the FBI's "hit list", Officers Richard Gylfie and Bayes violated my constitutional rights, which protect me as a photographer and against unlawful stops, searches, and seizures.
More on the video's youtube page.