City officials say they are pleased with the outcome of a massive disaster response training exercise that turned the rail yard and surrounding streets of Maspeth, Queens, into ground zero of a terrorist attack yesterday.

More than 1,500 emergency responders participated in the drill, codenamed Trifecta, which simulated a chemical device in a freight train exploding as it passed a Long Island Rail Road train carrying passengers.

Officials said the exercise was a test of the Citywide Incident Management System, which was developed to improve coordination among many agencies during a disaster.

“It worked very well,” the commissioner of the city’s Office of Emergency Management, Joseph Bruno, said. “The system was good. People took the right actions.”

The drill was meant to provide lessons to officials constantly reviewing and tweaking the city’s response plan, but neither Mr. Bruno nor the police commissioner, Raymond Kelly, pointed to any major flaws exposed yesterday. Mr. Bruno cited traffic flow among responders and the placement of vehicles and command posts as aspects that could be improved.

Officials designed what they said was a plausible situation in which a device filled with arsenic trichloride explodes on a freight train, sending a blast of hydrochloric acid into a passenger train. The mock explosion killed 26 and injured another 70, including three responders who got too close to the deadly chemicals and were “taken out of play,” a spokesman for the emergency management office, Jarrod Bernstein, said. (As a mental health precaution, officials do not actually tell participants they would have been killed, Mr. Bernstein said.)

Police shut down several streets around the rail yard, and emergency personnel arrived at the scene based on projected response times. Police cadets acted as victims in the exercise. They wore tags indicating their condition so they could be triaged by responders.

Mannequins also were used, and in the aftermath of the drill they lay strewn along the train tracks.

The mock attack involved agencies ranging from the FBI and FEMA to the city’s Department of Environmental Protection and the American Red Cross. The Police Department took the lead, and during the drill senior officials established a joint command post with the Fire Department and the Department of Environmental Protection.

A centralized command is a key feature of the city’s disaster response plan, following reports indicating that a lack of coordination contributed to the death toll among police and firefighters during the terrorist attacks of September 11, 2001.

Officials put the cost of yesterday’s drill at $700,000, paid for by a federal grant from the Department of Homeland Security.

The drill was the city’s largest field training exercise since May 2004, when more than 500 emergency personnel responded to a mock bombing at the Bowling Green subway station in Lower Manhattan. That exercise featured two non-chemical devices, one of which detonated.

The city publicizes its large-scale drills, but Mr. Kelly said there is no risk that useful intelligence could be handed to possible attackers.

Still, the official reports filed after the exercises are kept confidential, so as not to expose weaknesses in the city’s response plan.