At 2nd Ave. Sagas, a blog dedicated towards New York City transit infrastructure, has a recent article about the Proposed Subway Expansion between 1929 and 1939. Back then, there were plans for an even larger subway system connecting every borough and covering more area all over the city with 100 miles of new track.
At EV Grieve, a blog centering / lamenting on the East Village, a post: Story of a City, where the author has found some old films about the city from the late 40's.
P.S. The whole idea that the largest subway system in the world may still have more potential to better serve its city, is extremely interesting to me, so interesting that I've wanted to create another blog to gather all the information I could about New York's transit infrastructure and map it out to see if we are really served better now than before. All I've got is a page with some links because I just haven't found the time. If you want to explore all the links I've got up on the blank page it's here: Tracing New York