A monorail that’s not much more than US$10 million/mile.
Today, we were in Milwaukie for my father's 80th birthday party, and while we were there Russell decided he wanted to build something with the family L*g* blocks. So we built a monorail.
It's pretty simple to build a block monorail car, if you don't care about trivial things like curved track, power, a smooth ride, or carrying passengers.