The problem with having only car lanes is that they’ll always be congested, no matter how many lanes you add (look up induced demand). Trains have so much more capacity that we don’t run into this issue.

Basically, 4 car lanes=traffic jams twice a day vs. 2 car lanes + 2 train tracks= traffic flowing freely.

Of course a subway would be even nicer but those aren’t always an option because tunneling is expensive