The new York subway is used often as an example of how well the private sector can build infrastructure. But really, those two private systems (bmt, irt) where

The new York subway is used often as an example of how well the private sector can build infrastructure. But really, those two private systems (bmt, irt) where the original PPP. New York city actually backed those systems with municipal loans (bonds), which is a pretty big subsidy - and a setup of "private profits, public losses". In any case, the two private systems are badly designed relative to one another: bad transfers, redundancies. The IND built by the public is overall much better designed. The fact that both the private and public where able to build giant subway systems in the 10s-40s but it all fell apart after, should give some idea that it wasn't anything inherent to being public or private that allowed building it and made it difficult later, but other factors.