There is such a principle and even a law, cost/benefit analysis. And it doesn't consider your externalities. C/B analysis is especially simple for a train --

There is such a principle and even a law, cost/benefit analysis. And it doesn't consider your externalities. C/B analysis is especially simple for a train -- can estimate the benefit from the ticket revenue. If more is needed, then the train fails C/B analysis. But for the NY and NYC MTA, they don't care because NYC needs the trains. For the rest of the country, they should and do care. If you want my tax money for your new dreamy trains, then I will tell my representatives to vote no.