Taxi is to a bus, but what is the missing link, that is to trains? The reason that the missing link doesn't exist, isn't because of some economic rule that mass transit systems must abide by, but one of engineering.
Trains run every 10 minutes or so because of
Imagine we have two sensors, A sensor and B sensor, which are places 3 miles apart. Trains on this section of the track are designed to operate at 60 mph, so take 3 minutes to pass from sensor B to sensor A. If the train slammed on it's brakes at sensor B, it would take 1 minute for the train to stop, and it would do so on sensor A.
Now imagine there are two trains, Train X and Train Y.
Train X passes sensor B. Train Y is following but it can't enter this section ("block") until Train X has passed sensor A. Until Train X has passed sensor A, it could theoretically be *anywhere* between sensor B and sensor A, including right in front of sensor B.
So where is a safe pace for Train Y to be when Train X passes sensor B? Well it needs to the same distance again away from Sensor B (let's call it sensor C). And the same distance again away from sensor C (let's call it sensor D).
That way if Train X doesn't ever reach sensor A, Train Y will have only reached sensor C in that time, and it can slam on it's breaks and stop before it reaches sensor B.