When I was heading south, the Metrolink only goes to Oceanside. Don't know about Amtrak. I use the Metrolink app to find schedules and order tickets. Highly recommended!
Can I board the Pacific Surfliner at LA Union Station?
Use the metrolink app and check the website
Amtrak's Pacific Surfliner has 12 scheduled trips per day. Check their website.
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