As difficult as it is to believe, the New York City Subway has not always been the paragon of cleanliness, courtesy and efficiency currently enjoyed by several million New Yorkers and out-of-towners each and every day. In fact, 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, cars back in the day were relatively rickety affairs, and frequently sauna-hot in the summer. Crime on trains and platforms was not unknown. Sharp-eyed travelers might occasionally spot litter. And while contemporary commuters can, and do, set their watches by trains' arrivals and departures, the old subway's schedules could often, to the initiated, seem arbitrary—nonexistent, even.
Verily, New Yorkers today live in a mass-transit Golden Age. (Hint: Sarcasm.)
And yet, looking back, the messy, old-school subway system can sometimes feels more . . . charming. More human. Romantic, almost.
Fortunately, way back when, LIFE made a habit of chronicling the frenetic, surging underground scene (not to be confused with today's skinny-jeaned, sockless, complicated-eyewear, fedora-wearing underground scene). Through the years, the magazine's photographers routinely descended into the loud, dim underworld to document the singular sights found there. In the 1940s, '50s and '60s, the likes of Cornell Capa, Ralph Crane, Eliot Elisofon and other masters found on the trains, on the waiting platforms and in all of the other areas associated with the subway a universe unto itself, where the behavior of the people below ground felt similar to—and at the same time slightly, indefinably different than—behavior encountered topside.
Here, more than a century after the opening of the system's first underground line (long after elevated sections of the Interborough Rapid Transit, or IRT, system had been introduced), LIFE.com pays tribute to the Big Apple's vast, serpentine, almost incomprehensibly complex subway system; the army of men and women who, improbably, keep it up and running; and the intrepid souls who, every day, several times a day, in all kinds of weather, run down those well-worn steps — even though you know you're not supposed to run!—hoping to squeeze on to a crowded train in the last few seconds before the familiar, two-note "ding-dong" warning chimes and the doors, irrevocably, close.
Liz Ronk, who edited this gallery, is the Photo Editor for LIFE.com. Follow her on Twitter at @LizabethRonk.