Buffett Divulges Details on Berkshire Railroad Holdings

Billionaire investor Warren Buffett revealed the names of two railroad stocks recently purchased by his holding company Berkshire Hathaway, through documents filed with the U.S. Securities and Exchange Commission.

Berkshire said in the filing that it owns 10.5 million shares of Union Pacific and 6.4 million shares of Norfolk Southern.

Berkshire revealed an 11% stake in Burlington Northern Santa Fe in April, which made Berkshire the transportation company's largest shareholder.

The legendary investor told CNBC's Liz Claman last month that Berkshire bought large stakes in two North American railroad companies but declined to give names. In another recent interview with Claman, however, he did discuss his reasons for investing in railroads.

In a separate SEC filing, Berkshire said it increased stakes in two major health care companies, Johnson & Johnson and Sanofi-Aventis .

According to SEC rules, Berkshire would have had to quickly disclose a 5% stake had Buffett intended to use it to influence Burlington Northern management. The SEC requires companies to publicly disclose a stake of 10% or more.

Berkshire owns more than 10% of the outstanding shares in five other companies: American Express, Moody's, USG, Washington Post and White Mountains Insurance Group. The company's largest position in a publicly traded stock is a $9.7 billion stake in Coca-Cola.

The SEC allows Buffett and others to delay disclosure of purchases in order to prevent investors from piggybacking positions.