How's this sound: rather than hopping on the subway, when in a large city you'd hop into a little pod hanging from an elevated track. It would have pedals. You would then pedal yourself around the city, working up a lovely sweat before you reached your destination. Sound good? No?
Well, thank your lucky stars that the people who run our cities have at least some sense in their heads, because it would be highly doubtful for this to appear in the flesh anytime soon. Even still, designer Geoffrey Barnett built one for his New Zealand adventure park as a proof of concept, hoping to convince people to build them elsewhere.