I was just in Paris. It was a major eye-opener for me to observe people on the Métro when there was a train approaching while I was a little ways away from the platform. Very few people rushed to make it! It initially made me doubt that I understood how the train worked—did they know something I didn't, and this train wouldn't actually stop where I needed? But no, it was just that they knew that if they missed the train, they would only need to wait a whole 3 minutes for the next one.

Coming from the SF Bay Area, if you miss a train you'll probably have a minimum 15 minute wait (at least at the stations and times I use), often 2-3x more. People run down the stairs to the platform, shove their way in, and often block the doors from closing if they're squeezing in at the last minute (which extends the last minute, making the train later for the next stop.)

Also, "taking the train" doesn't just mean showing up at the train station. It means checking the schedule and timing it so you show up at the right time. I don't just walk from place to place, using the train as a way of getting around as needed. I plan it out. Or I don't, and use a car instead, which everything subtly encourages.

I agree, frequency makes a huge difference in actual practice.

For all its recent problems, the Boston subway isn't too bad. I certainly don't check schedules.

But the commuter rail out to me only runs about once an hour. I can deal with that for a "9 to 5" work thing where driving would have to deal with horrific traffic anyway. But it's a total non-starter for an evening event where I would have to time my return including possibly dealing with subway variance to a commuter rail station. (And it takes significantly longer than driving at that time of day anyway.)

And frequency is related to density.

The denser the population the better mass transit works and the more burden having one's own car produces.