As the FBI investigates the cause of the Amtrak derailment in Philadelphia, NJ Transit police conducted a drill Saturday.

The Office of Emergency Management, police officers and firefighters simulated a "mass casualty" incident at Newark Light Rail's Washington Street station. The drill involved protesters chaining themselves to a train amid underground fires. Sirens and thick smoke filled the air as responders assessed injuries and made arrests during the drill.

The training has been in the works for 18 months, officials say.

"It's very difficult to just train for something like this sitting in a classroom," said NJ Transit spokeswoman Jennifer Nelson. "It gives them a real sense of what it's like to deal with this kind of situation in real time."

Crews train for various situations -- such as hurricanes, derailments and snowstorms -- at different stations across the state.

Saturday's drill took about three to four hours to complete. Extra buses transported commuters during the drill.

NJ Transit is the third largest transit system in the country with about 165 rail stations and 62 light-rail stations.