See How They Run
I thought about this today while running for a train. D.C. strap hangers know how long it will be until another train appears, so when they see one coming — especially at the end of the Silver Line, where the tracks are visible across a vast stretch of elevated sidewalk — they take off.
This is in addition to the escalator and stair-climbing (systems are often broken so you'll be climbing no matter which you take), the balance improved by frequent standing in crowded cars, and, of course, hanging on for dear life (great for upper body strength).
The commuting life is a healthy life, as long as you ignore the stress levels. Take those out of the equation and you have the perfect fitness opportunity. Puts a whole new spin on the words "in training." Why join a gym when you have Metro?