It's magical when you can rely on public transport in a city to work during declared hours... You move without having to think about it, like fish in water. Any unplanned downtime always throws a wrench in the plan, if they are frequent then people will prefer private transport and clog the roads.

I suppose there are other reasons for redundant tracks (a train malfunctions and needs to stop somewhere while the next comes to serve?) and during the times they are in maintenance there is no slack.

Re ROI, I never thought it had to be positive for public transport... most wins from functioning transportation seem less tangible like increased public mobility (better for economy), reduced road congestion (=> less mortality while waiting for ER or police, better air quality), mental health etc. But I guess that's USA.