Even more interesting, in a move that highlighted Buffett’s bullishness on railroad stocks, Berkshire doubled its stake in Union Pacific Corp. (UNP), taking its holdings from 4.45 million shares at the end of March to 8.91 million shares as of June 30.

Last year, Buffett and Berkshire road the rails hard. Buffett made his first move on Burlington Northern Santa Fe Corp. (BNI) last April, acquiring nearly 40 million shares - or close to 11% - of the railroad. He then moved on to snap up 10.5 million shares of Union Pacific Corp. (UNP), and 6.4 million shares of Norfolk Southern Corp. (NSC).

Later in August, Berkshire went shopping again, loading on an additional 3.3 million shares of Burlington and another 6,000 in September. But Buffett didn’t stop there: He added yet another 10,300 shares of Burlington over the two-week period ending Jan. 22, bringing Berkshire’s total stake in the company to 18.2%.

Berkshire’s second-quarter acquisitions, which were disclosed in an SEC filing last week, are only a fraction of the $3.98 billion Berkshire spent on stocks in the
April-June period. Even if Buffett bought the shares at their highest second-quarter prices, which he almost certainly did not, the total cost would only have been about $260 million. That means more than $3.5 billion went into smaller amounts of unnamed stocks the company was not required to disclose.

Where that money went is anybody’s guess, but Buffett indicated in a recent interview with CNBC that a portion of it went into one of two stocks: Wells Fargo & Co. (WFC) or American Express Co. (AXP).

Wells Fargo stock has plummeted 22% in the past year, while American Express is down more than 37% in that time. However there may be some clues as to which stock Buffett really believes will rebound in some earlier comments he made.

“We’ll say at American Express… they are experiencing credit deterioration and they’re experiencing it sort of in all segments,” Buffett said earlier on CNBC’s Squawk Box. “So they’re seeing the rich customers slow down in payments, slow down in purchases.

“And American Express can describe that rather than I,” he added, “but I pay a lot of attention to that sort of thing. And incidentally, it will get cured at some time in the future, but right now the situation is getting worse and I would say that I don’t see any early end to that.”

That assessment doesn’t seem particularly favorable, particularly compared with comments Buffett made with regards to Wells Fargo just a few months ago. 

"Wells Fargo stock was down last year,” Buffett said, “I don’t think the intrinsic business value shrunk. In fact, I said I thought it probably increased a touch."

Berkshire already owns considerable stakes in both companies.