The company that owns Greyhound buses is nearing the end of a lease at its downtown station, and that could leave Chicago poised to become the largest city in the Northern Hemisphere without an intercity bus terminal, according to a new report. Closing the station, especially in a transportation hub like Chicago, would be felt acutely by low-income riders, residents of communities without easy access to train or airline service and others who rely on buses to travel between cities, advocates said.