Over the years we've looked back at the state of the subway throughout its many years underground in our Flashback series, and now LIFE is paying tribute to it with a look back at their photos from the 1940s through 1960s (when the cars finally got air conditioning). They write, "for several decades in the middle of the 20th century, what was then the world’s busiest subway system was actually something of a mess. Unlike today’s flawless high-tech marvels [editor's note: lol] cars back in the day were relatively rickety affairs, and frequently sauna-hot in the summer." They also note that many magazine photographers would travel down below during those decades to document the straphangers and the world beneath the busy streets.

Check out all of LIFE's subway photos from that time right here.