In U.K. En.glish we talk about fire engines. In the U.S.A they would be more likely to say a fire-fighting Truck. I have never heard of the word wagon being used in this context. The main place in the U.K. people talk about wagons is the railways or in old cowboy films!

I believe that, in America, they are known simply as "fire trucks," as opposed to "fire-fighting trucks." On "this side of the pond," we only ever use the term "fire engine."

The most common word is "fire engine"