About 14 minutes into the movie, there's a motor vehicle accident on the UPPER deck of the Tobin Bridge. Clearly for 1/2 a second or so, you can see a City Of Boston EMS Ambulance on scene.

This would be unlikely because the only way to access the upper deck of the Tobin Bridge is from Chelsea. Therefore, most likely Cataldo Ambulance Service would respond to that portion of the bridge. (I worked for both Cataldo Ambulance and Boston EMS, so I'd know this first hand.)

I do concede though, that it probably happened that way because they wanted Boston's ambulance in the film since the movie was based on a Boston story.