Everything has to be cost effective. And to the public representatives or politicians, also incorporate a lot more things to consider, and to the later, prestige needs to outweight any risk.
the first underground rail of London (and the world) let to forrington, throu a poor neighborhood, which was just demolished, without compensation. Not even anything like a public hearing.
There's few places today, where it is urgently needed. A lot of places consider it to reduces trafic on streets, or easy of travel or something like that. You don't have cities where transport is near impossible. London was that before the first tube. There was a considerable amount of poors that hadn't been in the country side. And traffic in London WAS worse than it is today.
i studied the Scandinavian methods of planning public transportation, and i can't describe how complicated every thing has become. And not due to laws, just due to the society and requirements. They need proof that it will pay to build it. Not "just" that it'll easy the lives of thousands.