Even with 100,000+ inhabitants at the time of construction, it seems to be tremendously expensive to build. At the same time, impressive engineering. For some reason, I imagine maritime design engineers could achieve this most readily, given that it appears to be a 'first article'.
The practically non existent land use is a massive savings. It runs down a long valley which is extremely space constrained and mostly sits over the river. By comparison the largest cost of road networks in cities is opportunity cost. Something like 20% of the surface of NYC is roads, that’s hundreds of billions of dollars worth of land and massive amount of lost property taxes.
IIRC the current form of Grand Central station in New York was built when they buried all the tracks leading to it underground, to free up for building what is now some of the most expensive real estate in the world.
