In an SEC filing a short time ago on his 77th birthday, Warren Buffett's Burlington Northern reveals it bought 845,000 shares of the freight railroad Burlington Northern two days ago, at a price of $79.85.  Total value of the purchase: $67.5 million.  Berkshire now holds just under 53 million shares, for a 15% stake.

The purchase reported in today's filing came just one day after the third of three days of buying disclosed in Berkshire's SEC filing on Monday.  That's when we learned Mr. Buffett had made a major purchase of just over 10 million shares starting the previous Thursday and ending that day, Monday.  The purchase Tuesday of under 1 million shares is relatively modest.

There had been two other filings earlier in the month. Back on August 17, Berkshire reported buying 1.4 million shares and on August 7 Berkshire said it picked up 1.6 million shares.

While Tuesday's purchase price of just under $80, Mr. Buffett stayed in the ballpark established by his earlier purchases.  But the stock has fallen since then, reaching an intraday low of $77.20 today.  It closed today (Thursday) at $78.21.  Current real-time price:

We'll see in the coming days if Mr. Buffett took advantage of the even lower stock price to add to his stake.  Berkshire is required to file its purchases with the SEC because it has a stake of more than 10%.

Please excuse me if you've heard this one before, but watch the rail stocks tomorrow.  But since Burlington is now below Mr. Buffett's repeated purchase price, we may not see a rally.  Remember, however, that he's not in this for the short haul!

Questions?  Comments?  Email me at buffettwatch@cnbc.com