Tram is about every 15-20 minutes & runs all day, as long as Universal studios hours. Tram is free but tap card cost $7.50 for the day. Enjoy your time at the park. Fun for all family.
You would take the Amtrak Pacific Surfliner from Irvine to Union Station. After that you would come from Union Station and buy a Metro Light Rail pass or TAP Card for the Red line that's going towards North Hollywood. The stop is Universal City. You'll go across the bridge and that is where you will wait for a blue tram where you will enter City Walk.
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