Just to share my experiences (and this is by no means the official stance):

I was there at night exactly two weeks ago. I was told by one security guard that I had to put the tripod away but that picture taking was fine. I asked another security officer about this upon entering the station. She said cameras are always fine and that a tripod was OK so long as I was willing to sign a release form. I figured better to not make a big deal over it, so I went without the tripod. I took pictures for over 2 hours and actually used some benches/concrete platforms as a tripod. While not the greatest of shots, it worked well enough, plus it was sort of fun trying to be resourceful. I had no problems whatsoever, and this included taking shots in front of Amtrak/Metrolink personnel as well as other security guards.