They might mean energy efficiency or time efficiency (reading a book etc on the train). Or maybe construction energy efficiency, which in Austin is presumably

They might mean energy efficiency or time efficiency (reading a book etc on the train). Or maybe construction energy efficiency, which in Austin is presumably a sunk cost. Note a 15-30 minute wait is what I have to endure at night (21.00 to 01.00) if I want to travel to/from the suburbs of Copenhagen on a weekday, similar population to Austin. On Friday and Saturday nights each line runs every 20 minutes all night.