This is a map of the Bay Area Rapid Transit network:

And this is an elegant visualization by Ray Luong of ridership levels over the course of one day: February 4, 2016. If you can’t see the embedded video below, click here.

[youtube https://www.youtube.com/watch?v=owGgbAS7Wq8?rel=0&w=560&h=315]

Note how the lines speed up as they go through the Transbay Tube connecting San Francisco and Oakland. That’s actually what happens. Within the 10 km-long tube, the trains reach ~130 km/h, which is more than twice as fast as the average speed throughout the rest of the network.