The repetition of the phrase "I came from" creates a rhythmic pattern in the lines. The repetition of this phrase also serves to emphasize the diversity of the cargo being carried by the box cars. Additionally, the repetition of the phrase "a mile long" throughout the lines serves to emphasize the length of the trains and creates a sense of continuity and movement. This repetition of phrases creates a rhythmic pattern that mimics the sound of a train moving along the tracks.