The second avenue line was a "major investment". It was also probably [many] decades after any private company might have done it.
When one have lived for decades with the same nationalized system, one can't imagine what the alternative might be like. But we know from other industries. E.g., telecoms. The UK nationalized its telephone industry in 1912(!). The rest of the world basically followed suit (the U.S. didn't, but we gave AT&T a monopoly that tended to limit innovation in the same sort of way), then in the 90s the whole world privatized and/or deregulated telecoms and... wow, what a boom resulted! And yet very few people who are aware of that case will even think of applying the same approach to other typically-nationalized industries.
It's always interesting to see what gets nationalized. It seems to be the biggest, most notable, locally culturally valued industry that the relevant politicians have the power to nationalize. National governments tend nationalize big things, like telecoms, utilities, steel manufacturing, etc.
In the U.S. the Federal government has not had the ability / constitutional power to do this (see the Steel Cases from 1947), the States have and have had the constitutional power, but by dint of having so many of them in competition with each other, they've mostly been unable to use it, which leaves: the cities. In the U.S. every city has a "nationalized" public transit system.
But in many parts of the world things like city buses are entirely private sector industries (e.g., in Buenos Aires, which has an incredibly vast private bus network). That might be surprising, since in Argentina just about everything of note has been nationalized at times. It makes sense though: Argentina has a strong central government, and weak local governments, so the central government nationalizes things of national importance, and the local governments not so much.