Living in the suburbs in the U.S., we hear about such things called urban mass transit. Things like trains and subways that spread out from downtown like the tentacles of an octopus. The closest thing we have in Houston are buses that no one wants to ride and a really nice train that only goes through the museum and medical districts. If you want to get to work, you own a car.
Below the streets of London and Paris however, is a labyrinth of tunnels and passageways. Like the tunnels of an ant farm, many people ride the “tube” or “underground”. Occasionally you find a few stations above ground, as in this photo from London, but most will be below ground, accessed by curbside stairways scattered around town. During rush hour, you better move fast, as tunnels and trains get crowded. Concentrating on their route, people have “tunnel vision” doing their best to get to their next connecting stop or final destination.