[bobthepanda]

In a rail network you are only as good as your most congested pipe. BART is a pretty good example of this, because while the outer legs are relatively infrequent, they all converge on the Transbay tube at a combined 2.5 minute frequency, which is about the safe braking distance with some buffer for a train of that size and weight. Longer convoys would require even more braking distance, and already trains cannot rely on the AV technologies of cars because the braking distance is beyond line of sight.

You could run more trains only between the suburban branches but there isn't a whole lot of demand for that.