The subway B line is finally back up and running....

Straphangers suffered through a nightmare commute Friday morning when a power outage at a Midtown subway station led to hours-long suspensions, delays and rerouted subway lines.

Gov. Cuomo, who last week called on Amtrak to be transparent about the state of its aging infrastructure, pinned the blame for the chaos on Con Edison — which accepted responsibility.

“Simply put, this was completely unacceptable,’’ said Cuomo, adding that he has ordered a state ­investigation.

The commuter chaos began 7:25 a.m. The worst of it was over by 1:30 p.m., and normal service ­finally came back around 5 p.m.

The trouble began when power that moves the trains, activates signals and provides lighting at the Seventh Avenue-53d Street station went dead.

The station is served by the B, D and E trains.

That paralyzed not only that station but most of the “lettered” lines throughout the system.

Fatima Callum, 23, who was stuck on an E train for more than an hour, was furious.

“Everybody was late for work. Everybody was just miserable. There was nothing that we could do. We were already underground — no service.”

Many riders have come to ­expect chronic delays and cancellations.

Before Con Ed admitted it was responsible, subway users vented at the MTA.

“The MTA, even on the weekends, is always crazy,” said ­Anthony Warrington, 56, who was desperately trying to make it to a 1:30 p.m. job interview.

“We spend so much money for this and get hardly any real service. It tells you how the infrastructure is.”

Con Ed declined to say what caused the outage.

Riders urged the MTA to modernize the system.

“The MTA needs someone new — maybe fresh eyes to look over what could be implemented in this situation,” said a rider who gave her name as only Judy D, 36. “We shouldn’t have to deal with this.”

Grace Guarnieri, 21, of Harlem, said one outage shouldn’t bring the entire system to a grinding halt.

“It’s really poor planning,’’ she said. “To get home, it’s just a nightmare.”

Additional reporting by Stephanie Pagonesand and Elizabeth Rosner