An interagency training exercise involving more than 400 police and fire personnel from King, Snohomish and Pierce counties was held in Seattle's Sodo neighborhood Sunday afternoon.

A handful of passengers, retching and choking, stumble off an Amtrak train as it pulls into Seattle.

When firefighters arrive, a conductor tells them the passengers in one car seem to have been exposed to some hazardous gas.

Turns out it’s sarin — colorless, odorless and deadly.

As firefighters treat and decontaminate the victims, they learn there are dozens more still on the train — along with a suspected terrorist, wearing a gas mask and potentially armed.

Then comes word there’s another suspected terrorist, also wearing a gas mask, also probably contaminated, who got off the train and is now several blocks away.

This was the scenario that played out Sunday in Sodo in an interagency training exercise involving more than 400 police and fire personnel from King, Snohomish and Pierce counties.

Amtrak provided the train. Fire cadets played the victims. Responders — paramedics and members of hazardous-materials and SWAT teams — played themselves.

“They don’t know what the drill’s about,” Seattle Fire Department spokesman Kyle Moore said before it began.

“Victims” were washed down with fire hoses. A black armored SWAT vehicle rolled up to the train, disgorging team members who flooded the car where the terrorist was holed up.

Observers, identified by yellow vests, tracked and recorded the action, based on an all-too-real event: a sarin attack on the Tokyo subway in 1995 that killed 13.

The drill, two years in planning, was funded with a $300,000 grant from the federal Department of Homeland Security.

“The lessons that are learned here can be used nationwide,” said A.D. Vickery, a Seattle Fire Department assistant chief.

Among other things, the exercise’s planners wanted to see how effectively professionals from different disciplines and departments worked together when presented with a complicated, fast-changing, dangerous situation.

“You can’t just have all these first responders showing up without a plan,” Moore said. “You can’t just wing it.”

The drill will be documented and analyzed in a report that should be finished within the next two months.

“Hopefully, we’ll identify some gaps, things we need to improve on,” said Paul McDonagh, a Seattle Police Department assistant chief.

Eric Pryne: 206-464-2231 or epryne@seattletimes.com