MBTA, Green Line (light rail) vs either Red, Orange, or Blue Lines.
http://www.mbta.com/uploadedfiles/do...Edition(1).pdf
Some data that should surprise you.......FY 2013 data.....
Green Line daily ridership 227,645
Red Line daily ridership 272,684
Orange Line daily ridership 203,406
Blue Line daily ridership 63,225
The Green Line, Boston's light rail line, stands up very well against Boston's other heavy rail lines individually.
That's just one city, all that you asked for.
And by the way, heavy rail reaches a capacity limit, the shorter the headways the shorter the trains. There's a reason why most heavy rail trains are limited to six cars. Start adding more cars and the headway must go up.
It's the terminus station setup that limits headways to around 2 minutes; time for passengers to alight and board the train, and time for the operator to switch ends and cabs. The longer the train, the longer it takes the driver to switch ends and cabs.