Think of it as the homeless hotspot thing from SxSW, only brought to the peninsula. One person per train would probably do the trick.
The hardest part is: just how do you get reliable connectivity to your box in order to resell it?
Summary: uses a bunch of different 4G cards from different carriers, large high gain antennas, and link aggregation; to make a single reliable network connection from a bunch of unreliable ones.
Downside: Fills a backpack, is really thrillingly expensive.
I imagine you could probably test the waters by running an open AP with a suitable name and have a small web server behind it which lets people indicate their interest. Ride with it for a few days, wear the name of the network on your clothing, and see what happens.