The first line in Moscow was opened in 1935. Most of the present systems in London, Paris and NYC were in place a decade earlier. Paris and NYC started -- with rapid expansion -- at the start of the 20th century, London even before.

After WWII, Paris, Moscow and New York kept extending old lines and building an entirely new one every few decades, and even London built a new line into the nineties. But New York more or less stopped in the early seventies. The one big project today is the Second Avenue Subway, put on hold repeatedly since the early fourties(!), but now in construction at last. To be fair, New York also built less spectacular but capital-intensive connectors and track quadruplings, the last one being the 63rd Street Connector.

_{*Lunatic*, n.
One whose delusions are out of fashion.}