The Chicago SunTimes has an article on the 5-11 Club Canteen Service

As water misted off firehoses, splashed from puddles and froze on the gear of firefighters early Wednesday at an extra-alarm blaze in the South Loop, Russell Harmon stood by with a steaming pot of coffee. Harmon, 58, got a page about 3:30 a.m. and left his West Loop home minutes later. He’s part of the 5-11 Club, a group of volunteers who show up to major fires to hand out coffee and hot chocolate to firefighters and other emergency personnel.  Four 5-11 Club volunteers arrived in one of the group’s two trucks, which are stored at Chicago firehouses. In the summer, they hand out cold drinks. The club derives its name from the call sign reserved for the most severe fires. They will arrive at all fires that are 2-11 and above.

The temperature hovered at 7 degrees with subzero wind chills.

“We went through four pots of coffee and a couple hundred cups of hot chocolate pretty quick,” Harmon said.

“We pulled up and there were 30 guys waiting for something warm,” said Jim Mitidiero, a fellow volunteer. “A lot of these guys have icicles on their gear … by the time we got there, they’d been fighting the fire for more than an hour,” Mitidero said.

“I tell people people, ‘The colder it gets, the better my coffee and hot chocolate tastes,” joked Mitidiero, a retired Chicago police officer.

“You hand them something warm, and they’re very, very thankful. But we just want to say thank you to them for putting their lives on the line, so it’s weird to hear them say thanks,” said Harmon, a retired CTA switchman.

There are about 50 volunteers from all walks of life in the group, but only 10 who regularly operate the trucks, more commonly known as canteens.

… the truck, in a previous life, was used by the fire department’s scuba team.

thanks Dan