The secret world behind New York City's subway, which millions of people use every day

AP

The New York City subway system is one of the most iconic in the world, servicing over five million people every weekday. But its also been mired in budget problems, and some parts of its technology have remained largely unchanged since its inception in 1904.

The combination of these two things makes working on the subway quite the task for the Metropolitan Transportation Authority (MTA). And the MTA's Instagram does a good job of showing what it's like, taking you behind the scenes of flooded tunnels, snowy tracks, and welding jobs.

Here are 24 photos that show what it takes to keep one of the world's biggest subway systems running:

Additional reporting by Graham Rapier.

"Crews are preparing to scrape old paint off the tunnel ceiling at 59 Street on the 4/5 lines..."

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"There wasn’t a cloud in the sky, so MTA New York City Transit had to bring in their own water to perform a 'wet test' of the flood barrier protection at the Bowling Green Station in Lower Manhattan."

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"Here, an old section of the third rail is hoisted up onto a work train near Classon Av..."

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