Norfolk Southern Corp. announced late yesterday that it has purchased "for investment purposes" 5.01 percent of the common stock of Santa Fe Industries Inc., a move that would give the giant southeastern railroad a leg up in the creation of the first single-owner, coast-to-coast rail line.

The Norfolk Southern statement was issued without comment by a spokesman in Washington, who declined to answer questions. Securities laws require some statement when one company acquires more than 5 percent of another.

Railroad sources regard the stock purchase as a significant step toward a possible merger between Roanoke-based Norfolk Southern and a Santa Fe subsidiary, the Atchison, Topeka and Santa Fe Railway Co.

Federal officials said that the possibility of a merger would substantially increase the interest of other western or southern railroads in acquiring Conrail, the federally owned northeastern freight railroad that the government wishes to sell back to private investors.

Andras Petery, a transportation analyst for Morgan Stanley who closely follows Norfolk Southern, said that one of the effects of railroad deregulation is that "you have to control territory and serve vast areas" to remain competitive, and Norfolk Southern's purchase "is obviously a 5 percent foot in the door."