"The oil from the Bakken, and from the Eagle Ford as well, turned out to be more volatile than people anticipated. ... We've lowered the speeds. It's going to require another kind of tank car, too."
BNSF announced previosuly that it will be buying 5,000 new tank cars. "We have found in the last year or so that it's more dangerous to move certain types of crude certainly than was thought previously."
(Read more: Buffett's Berkshire slammed by S&P)
In late December a North Dakota town had to be evacuated after a BNSF train carrying Bakken crude derailed and caught fire.
Buffett: Bitcoin 'isn't a currency'
In response to a question, Buffett said bitcoin "isn't a currency" and he "wouldn't be surprised" if it's not around in 10 or 20 years. Since the price keeps moving around so dramatically, "it's very speculative, a Buck Rogers kind of thing" like the Dutch tulip mania in 1637.
Buffett on IBM and Heinz
Buffett conceded he knows less about major Berkshire holding IBM than he knows about Wells Fargo, one of his banking favorites. He knows enough, however, to feel comfortable about owning the stock and IBM's move lower means "nothing" to him. He conceded that IBM's revenue trends have been below expectations, but he likes the company's stock buybacks. He also endorsed CEO Ginny Rometty. "I feel fine with her, but her record will be judged five years form now. There's a lot going on in that business."