If not a subway at least some form of transport that won't get stuck in gridlock. Three examples I can think of are the Wuppertal suspension railway, Las Vegas monorail (though more of a tourist pull than for the locals) and the dedicated Metrobus lanes in Mexico City.
https://en.wikipedia.org/wiki/Wuppertal_Schwebebahn
https://en.wikipedia.org/wiki/Las_Vegas_Monorail
https://en.wikipedia.org/wiki/Mexico_City_Metrob%C3%BAs
What others have I missed?
(btw I never noticed before that HN can't handle UTF-8 chars in urls)