Fresh off a massive $26.4 billion railroad deal, Warren Buffett is praising the U.S. economy for its resilience and growth potential. In a speech to Columbia University business students Thursday, Buffett gave both Federal Reserve Chairman Ben Bernanke and Treasury Secretary Tim Geithner "high marks" for handling the financial crisis, which he said had left its most dangerous phase. "The financial panic is behind us," Buffett said, according to Reuters. "Our economy was sputtering, still is sputtering some." The billionaire explained Berkeshire Hathaway's purchase of railroad company Burlington Northern Santa Fe Corp. as based on expectations of population growth and stricter environmental controls. "There will be more people in this country, 10, 20, 30 years from now," Buffett said. "They'll be moving more and more goods back and forth to each other and the most environmentally friendly and cost-efficient way of doing that is railroads."