You may recall that, up until 2004, Amtrak hauled bulk mail for the United States Postal Service. In 2004, however, Amtrak canceled the contract with the USPS because the Amtrak leadership of the time believed passenger service was being disrupted by the mail service (with switching, etc). So, nearly overnight, Amtrak stopped the practice, much to the surprise of the USPS. After Amtrak stopped hauling mail, the boxcars they had owned for that purpose were sold off. Every once in awhile, you might catch one of those boxcars, still in Amtrak's Phase IV scheme, on the rails!
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