Taipei say they need 3:30/night for maintenance i.e. 24.5 hours/week. That lines up pretty closely with London's 5:00/night 5 nights/week, i.e. 25 hours/week. So I could believe that NY needs to shut each track pair for 25 hours/week to do adequate maintenance. But they can achieve that just as well by shutting one track pair for 7:00 each night rather than both track pairs for 3:30 (indeed you can probably get more done in one 7:00 block than two 3:30 blocks).

True, but that is assuming Taipei doesn't have extra tracks factored in...

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.

New York has the only consistently 4-track subway system in the world; they have convenient tunnelling geography and built it back when life was cheap. The ROI on building a new 4-track system would never stack up today (I suppose you could argue that it doesn't stack up for NY either in terms of the cost of doing twice as much track maintenance, but I believe they do use all the capacity during peak hours so it's not like there's really a business case for dropping down to 2-track).

Interesting!

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.