The heat stay packed like the train on Monday
Generally the trains in the northern part of Chicago are usually packed… especially on Mondays, since many people go to work that day. Therefore the mass exhalation of carbon dioxide makes the train hot in temperature.
Also referring back to the first line, Lupe makes a metaphor for his mind. He uses the phrase “packing heat” to state that his mind contains a lot of hot rhymes that he will shoot (so to speak).
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