His nickname comes from earlier train-hopping. The text adds that he carries a machete and makes shoes. The details are given with the casual density of an already-known local eccentric rather than a conventional introduction.
Boxcar's main function is oral transmission: he arrives carrying a story about a Las Vegas casino, a pig farm and a grotesquely practical mechanical solution. He therefore belongs to the corpus's class of figures whose principal possession is anecdotal infrastructure—people who make the world larger because they have been somewhere else and come back with evidence too strange to verify cleanly.
See EATEN OR OTHERWISE and Last Trenta — Thirty Internal Works.