Adding more smaller trains without a fixed schedule over a broader area is exactly what they're doing. These smaller unscheduled trains are called "cars."

Running empty high capacity vehicles over a really large number of routes is immensely wasteful, much more wasteful than on-demand vehicles.

Standing for 14 minutes in the cold pissing rain is not fun. And that still doesn't solve the problem of getting to and from the train station.

And even that is ignoring how ridiculously expensive it would be to run trains every 15 minutes for the 6 people a day that have to get from West Bumblefuck to East Nowhereville.