From another article. "UPS is also the largest corporate customer of the railroads, shipping trailers and containers from U.S. ports to inland terminals where they are attached to trucks and delivered to customers around the country."
For example: The Chicago to Los Angeles trains are often exclusive UPS. UPS is/has been frustrated by the lack of infrastructure growth in the US to speed up train delays.
All part of a multi mode logistics + service provider for customers. There is a method to the madness. Domestically, the airline is only for what they cannot put on team drivers (PHL-LAX in about 58 hours), trains, regular truck routes (that's what they are<g>) and still make service gates depending on product. It bodes well for UPS to be so integral to these companies success as per the article.