@mtanyctransit delays have become a frustrating fact of life in New York City. Every day brings a new set of alerts about painful disruptions across the system. Trains creep along or stop in the tunnels. Crowds thicken on the platforms. Is the main culprit behind delays the aging equipment in the 112-year-old system — signals and tracks taxed beyond their limits and patched together, only to break down again and again? That’s certainly an enormous problem, but it’s not the No. 1 reason. The major cause of subway delays is a factor that basically didn’t exist 15 years ago: overcrowding. Subway ridership has risen dramatically since the 1990s, when about 4 million people used the system each day. Now it's nearly 6 million riders each day — the highest level since the 1940s. Those extra riders are cramming onto a system that's essentially the same size it's been for decades. But how exactly does overcrowding cause delays? Subway officials say trains slow down as they face an onslaught of passengers. “Dwell time” — the period a train spends in the station loading and unloading — balloons. And after a train starts to pick up large crowds, it becomes harder for people to get on and off, says Brussard Alston, who's been a train operator for nearly 2 decades. Then the delay reverberates down the line as a queue of trains behind it backs up. “Once the damage is done," Brussard says, "it takes a while for it to dissipate." @jakenaughton took this #timelapse video at the Union Square subway station in Manhattan. Visit the link in our profile to learn more about how subway delays happen.