(CBS News) Every major city has plans in the event of a terrorist attack. This past weekend, emergency responders in Philadelphia were put to a surprise test.
It could make other cities wonder whether they're prepared for a potential terror attack.
From the start, the idea was to keep the responders on edge. In fact, it was called "Operation Edge."
The responders got the information one piece at a time, just as if it were a real incident unfolding.
Even the commanders didn't know what the drill was for until they got there.
It began in the early hours of Sunday morning.
A "bomb" went off on a subway train at 8:30 a.m., trapping it in a tunnel.
Frantic calls flooded 911.
In one, a distressed woman was heard saying, "Help, help. I'm in the subway!"
In another, a man says, "Listen - I'm in the subway. My God, I can't see anything!"
A bit later, the woman from the first 911 call says, in a calm voice, "This is a part of Operation Edge. Do you understand that this is an exercise?"
Then, the "wounded" man also asks, also in a calm voice, "Do you understand that this is an exercise?"
But in a real crisis, would the plans work?
The small group that designed the drill disclosed almost nothing to the first responders who participated. They didn't even know it was a bombing in the subway until they arrived at the scene.
"We kept the information hidden from the main players, including myself, as to the specifics of the emergency that was going to be simulated," Philadelphia Police Chief Inspector Joe Sullivan says.
Almost immediately, problems surfaced.
At the command post, communications issues arose between departments. Underground, confusion over a life-and-death issue: Should the rescuers follow procedure and retreat until the bomb squad handled the other suspected IEDs (improvised explosive devices)? Or risk it and rescue the victims while the bomb squad worked?
The bombs and the victims weren't real, but the pressure and the decisions were.
They call it "stressing the exercise" - building the pressure on the players as the scenario unfolds, having the unpredictable event introduced into the scenario. Building enough pressure so that things will go wrong . . . because, in real life, that's the way it goes.
"It's very easy to carefully script an exercise like this and provide that script to all the participants in advance," Sullivan noted. "And we can come out here and we can go through the script and at the end of the day pat each other on the back and say what a great job we did -- and have learned nothing."
The designer of the exercise, bomb squad commander and Philadelphia Police Lt. Tom Fitzpatrick, added another wildcard, and another lesson learned.
Near the command post, an out-of town ambulance went un-noticed. Inside: a 900-pound bomb meant to kill the first responders. It was an important element borrowed from a real-world terror plot.
In fact, it was in London in 2007 that a car bomb was found right where police had set up their command post after responding to a terrorist incident.
Two years earlier, the London subway bombings killed 52 people. The Madrid train bombings in 2005 killed nearly 200.
Everything in the Philadelphia drill had actually happened somewhere.
In the end, not everything went according to plan, but from the start, that was the plan.
"It gives our guys a chance to see what went wrong ... so there's a lot of lessons learned today ... and to use it as a training tool," observes Philadelphia Fire Department Battalion Chief Anthony Hudgins.
Police Chief Sullivan said, "We faltered along the way, but we regrouped, and we worked our way collectively through those problems we were confronted with. And that's the way things happen in real life."
To see John Miller's report, including dramatic scenes from the drill, click on the video in the player above.