Warren Buffett's stake in Burlington Northern Santa Fe has reached 22.4 percent, with a new options-related stock purchase of over two million shares. In an SEC filing last night, Buffett's Berkshire Hathaway says it bought 2,325,000 shares of the freight railroad for $75 each last Friday, January 30. That's a total of $174.4 million. Burlington Northern closed at $66.25 on that day, but Berkshire didn't buy the shares on the open market.