STL Transit on Youtube has provided visualizations of several transit systems, with each vehicle represented by a moving dot. Time is sped up considerably, showing a 24-hour period in a few minutes
Here is Metrolink: Look for the express trains on the San Bernardino and Antelope Valley Lines, and the last few trains of the night.
And this is Orange County Transit Authority. You’ll especially note the busy core lines (#43 Harbor, #50 Katella, #57 State College and #60-Westminster), as well as activity along Pacific Coast Highway and I-5 to South OC. Watch out for the commuter runs going to/from Los Angeles or Riverside Counties:
Check out the STL Transit site for a few US and international cities. (Hopefully they will consider doing LA Metro soon!)