Chicago now has a few ALS truck companies. There is a chance that there will be an engine, truck and ambo on various ems runs. They will send the closest ALS company (either engine or truck) plus an ambo based on the nature of the run and the closest. If there is a BLS fire company (first responder) closer to the incident then an ALS company, both will be dispatched in addition to the ambo. It sounds in most cases it's one fire company and an ambo, but there will be times to get all 3. I'm sure Steve can elaborate more, but I believe this is the cliffs notes version.
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