The NYC subway system was mostly privately constructed by 3-ish private-ish organizations, and later joined under a state monopoly. You can still see vestigial

The NYC subway system was mostly privately constructed by 3-ish private-ish organizations, and later joined under a state monopoly. You can still see vestigial underground signage that says stuff like "To IRT Lines" or "BMT Lines -->," referring to now-defunct private subway operators. This is also why the rolling stock and tracking are visibly different from line to line.