Ideally: there's a train close enough to walk, or a bus or tram that's nearby that runs frequently, is clean, and doesn't get stuck in traffic because there's not much car traffic.

Slightly more realistic: enough people can and do cycle or walk to the train that pressure is relieved on the roads for those who cannot cycle or walk.

> And how do you get to the train when it's too far to walk and you're not a cyclist?

You get the bus, or you cycle, which is a life skill any able-bodied adult should have, not limited to cyclists. Of course not everyone is capable of cycling, but not everyone is capable of driving either.

A person living in DC can take the subway to Union station, take the Acela line to NYC, and then take the subway to their final destination.