I want to test the ability of students to read a passage and answer questions about it. Could you please come up with a good question for the passage "Philadelphia is also a major hub for Greyhound Lines, which operates 24-hour service to points east of the Mississippi River. Most of Greyhound's services in Philadelphia operate to/from the Philadelphia Greyhound Terminal, located at 1001 Filbert Street in Center City Philadelphia. In 2006, the Philadelphia Greyhound Terminal was the second busiest Greyhound terminal in the United States, after the Port Authority Bus Terminal in New York. Besides Greyhound, six other bus operators provide service to the Center City Greyhound terminal: Bieber Tourways, Capitol Trailways, Martz Trailways, Peter Pan Bus Lines, Susquehanna Trailways, and the bus division for New Jersey Transit. Other services include Megabus and Bolt Bus."? 

As of 2006, what is the busiest Greyhound terminal in the United States?