More U.S. railroads, less Chinese energy.

That's the trend in just five words based on the latest filings from Warren Buffett's Berkshire Hathaway.

Late Friday, we learned that Buffett's stake in Burlington Northern Santa Fe topped 17% as it exercised almost 8 million call options to buy shares.

Today, the news is from Hong Kong: a filing with the stock exchange there reveals that Berkshire's stake in the Chinese government-controlled energy company PetroChina dropped below 7%, as of September 21.  That's down from 11% in mid-July when Berkshire began a series of sales that now totals 860 million shares, worth well more than one billion dollars.

Under Hong Kong rules, Berkshire only needs to reveal those sales ("events") that take its stake across a whole percentage number. Intermediate sales don't have to be individually revealed.

Each filing, however, does list Berkshire's total PetroChina holdings after each reported sale.

By looking at the "event" dates in the filings and the total number of PetroChina shares held following each one, we can plot the average rate of sales.

Clearly, September saw a lot of sales and there could be more, since there's are substantial delays between the event date, the filing date and the date we get to see the filing. (We only learned of the September 21 sale today, about two weeks later.)  The Wall Street Journal reported earlier this month that Berkshire has been sending its disclosures to Hong Kong by regular mail, as it is permitted to do.  That has added some momentum to efforts to require electronic filing rather than simply make it an option.

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