The power went out on Friday in a busy place at an inconvenient time: a subway station in Midtown Manhattan in the thick of the morning rush.

Delays quickly cascaded outward from Manhattan, halting trains underground many miles from the site of the power failure and stretching normally routine commutes into hourslong slogs. At least a dozen subway lines, including many that do not pass through the affected station, were delayed.

It was another reminder of how the region’s infrastructure problems can leave overburdened transit systems vulnerable. But it also raised a question: How could a seemingly routine episode at one Manhattan station snarl so much of New York City’s vast subway system?

The Power Goes Out

The power failure occurred at 7:30 a.m. at the Seventh Avenue station, at 53rd Street. Because it knocked out the station’s signal system, most of the trains that stop there, on the B, D and E lines, had to be rerouted to other lines. Gov. Andrew M. Cuomo said the power loss had resulted from a Con Ed equipment failure.