It isn't likely that he dreamed, but if he did it might have been of being tied to the handle of a trunk in an overland limited baggage car; of the train's stopping for water at a lonely tank; of the earthy, wholesome country smell that came through the door, left open for coolness.
There had been a stirring in the grass near the track.

A glimpse of an animal that looked something like a fox and something like a wolf, and wasn't either one, a wild animal that was sneaking around the train for the odd bits of food that were sometimes left in its wake.

As the pungent scent of this beast reached the bulldog's snub nose, the leash that held him to the trunk became a thing of little worth.

With a violent lurch he broke it, leaped from the door, landed sprawling alongside the track, and was off in pursuit of the strange animal.
Now, any one who knows how a bulldog is built and how a coyote is built can imagine how much chance the first has to catch the second.

The dog followed by sight, not by scent.