Berkshire Hathaway, the investment company run by legendary investor Warren Buffett, has accumulated more than 10 percent of the Burlington Northern Santa Fe, the second-largest railroad in the U.S., behind Union Pacific.

The stake, which makes Berkshire the single largest shareholder in Fort Worth-based Burlington Northern, was worth $3.2 billion last week. But that is likely to change because other investors tend to pile into stocks Buffett anoints as worthy of being owned.

Berkshire disclosed its interest in Burlington Northern late Friday, in a filing with the Securities and Exchange Commission. In it, Berkshire said several insurance companies it controls -- Columbia Insurance Company, National Indemnity Company and National Fire & Marine Insurance Company -- had acquired more than 39 million shares of the railroad company.

Buffett has not disclosed why Berkshire is interested in Burlington Northern. Among its most profitable routes are those that serve the Powder River Basin in Wyoming and Montana, the world's largest single deposit of low-sulfur coal. That fuel still provides the largest single source of electricity in the U.S.

Burlington Northern serves 28 states in total, and is also a major shipper of corn and other agricultural products. It may also be poised to benefit from the growing interest in ethanol as a motor fuel. Plans are to produce the fuel close to where its main ingredient, corn, is grown, and then to ship it to major markets.

The railroad owns about 23,000 miles of track.

In a letter to Berkshire shareholders in February, Buffett said his company owned at least $700 million in common stock in 19 companies. He declined to name two of them "because we continue to buy them," he said. The filing on Friday identified one of the two.

Reuters reported that the S.E.C. lets Buffett delay disclosing Berkshire's purchases so investors cannot try to mimic them. Normally, S.E.C. rules would require Berkshire to disclose when it amassed a 5 percent stake in any company if it intended to influence the company's management. The rules require Berkshire to disclose a 10 percent stake even if it plans to be a passive shareholder.

In its annual report, Berkshire named five companies in which it held at least at 10 percent stake at the end of 2006. They were American Express, Moody's, USG, Washington Post Company, and White Mountains Insurance Group.

Its largest holding by market value, at $9.65 billion, was Coca-Cola.