OK, it's not a subway. At all. But it is a train. And from Atwater Village it's a painless walk to the Metrolink station in Glendale.
At the other end, in San Clemente, the tracks are practically on the sand. Travel time: two hours.
A train to the beach in Santa Monica could take 10 years longer. And cost $4,799,999,989 more.
To get zen about it, don't change how you go to the beach in Santa Monica. Go to the beach in Orange County.