It's a chicken and egg problem.

Our city nightlife is structured to wrap up around midnight, so why would we run empty trains in the middle of the night?

Our trains all end at midnight, so why would places stay open later than that?

I honestly think extending T hours is the wrong first step. It's expensive, poor service, and has big trade offs for maintenance. The easiest thing we can do is stop arbitrarily making businesses close early. If a taco place wants to serve until 4am let them.