Rapid and frequent transit is a matter of planning, investment and demand. You don’t need to have a particularly large population to sustain it.

Madrid for example is a city of about 3 million in a metro area of 6 million. Trains come every few minutes throughout the day.

> I would hasten to point out that not everyone walks or can take all public transit (BART elevators frequently being broken is an issue for wheelchair users, for example).

That is a BART problem, not a “public transit is inaccessible” problem.