I often say if I ever became rich I would extend the subway system, like having a line go across the Bronx and end up at Jamaica Center. Or maybe finally bring service into the huge swaths of Queens without any. Or maybe just put some elevated lines underground so they're no longer creating a nuisance on the street down below. But doing so would be a pain in the ass because now there are people living in those places. Major streets would have to be closed to dig them up. Parts of sidewalks would be requisitioned to create subway exits. See, in the early days the subways actually developed those areas simply because people could now actually travel to them. Many places were just empty before. Here's a picture of the 207th street station on the 1 in the early 1900s:

There was nothing in my fucking neighborhood before! It was literally dirt roads and grass! So building a train line wouldn't be an issue because you're not bothering anyone.

But what about downtown where it was developed even back then? It did encounter many of the problems that I mentioned above. In the 1910s the IRT line was being extended into the Village and at the time a five-story apartment was in the way of construction. Using eminent domain, the city took control of the property and tore it down. The owner David Hess was only left with about 500 inches of land, which the city asked him to donate. Considering he already felt fucked over, he angrily refused and created this mosaic on the small bit of land he had left:

You can see it now outside of the Sheridan-Christopher Street station on the 1 train, a testament to one pissed off man.