Most of the humor contributed by the language is contained in the dialect of the expresssman, who is described by the narrator as "a plain man of fifty, with a simple, honest, good-natured face, and a breezy, practical heartiness in his general style." The expressman remains in character throughout the ordeal. He never complains or blames the narrator for bringing such a smelly box aboard the train. An example of his speech is:
"I've carried a many a one of 'em,--some of 'em considerable overdue, too,--but lordy, he just lays over 'em all!--and does it easy Cap., they was heliotrope to HIM!"
The expressman resembles Simon Wheeler who does most of the talking in Mark Twain's "The Celebrated Jumping Frog of Calaveras County." Both characters are just naturally funny without realizing it. Mark Twain had a genius for creating such dialogue. He does it throughout his novel Huckleberry Finn, and Huck Finn the narrator tells the whole story in his amusing rural Southern dialect.
The expressman is especially funny because he doesn't like to complain about the smell out of respect for the dead man and for the narrator whom he assumes to be a mourning relation.
Twain uses dramatic irony to create humor in his story. The reader knows that a bag of cheese was left on the box, but the two men in the train car do not. Their confusion about the smell is humorous because they attribute the smell to a rotting body while the reader is aware that it is the Limburger cheese that smells so bad. The men in the car go to outrageous lengths to try to cover up the smell, even to the point of smoking cigars. When the heavy cigar smoke does not mask it, they restort to burning things. The finally end up riding outside the train car and catching pneumonia to get away from the small that if they'd only investigated, would have disgarded the bag of cheese.