New York City's World Trade Center site is set to be swarming with sirens and hundreds of police officers and firefighters this Sunday morning, but New Yorkers are being warned not to panic after last month's Air Force One photo op fiasco triggered fear and memories of 9/11 in many eyewitnesses. It's all part of an emergency response drill to simulate a bomb explosion in the PATH light rail system that goes under the Hudson River between New York and New Jersey and has its most important New York stop at the site of the World Trade Center.

Over 800 emergency responders will participate in the training exercise, which the city says is the largest drill of its kind in recent memory.
World Trade Center Set for Emergency Bomb Blast Drill Sunday - ABC News