W. Graham Claytor Jr., former chairman of Southern Railway, was named chairman and president yesterday of the National Railroad Passenger Corp., commonly known as Amtrak.

Claytor will succeed Alan S. Boyd, who resigned effective July 2. Although he declined comment on his future employment, Boyd, 59, is expected to be named soon as chairman and president of Airbus Industrie of North America, the U.S. marketing arm of the European aircraft manufacturer.

The Airbus post has been vacant since 1979. It had been held by George A. Warde, who then became Airbus' senior vice president of marketing before leaving the company last year to become president of Continental Airlines.

At a press conference yesterday, Claytor, 70, said he was dedicated to maintaining a national rail passenger system and the Boston-New York-Washington Northeast Corridor project.

"Amtrak made tremendous strides under Alan Boyd, and I propose to keep those strides moving," he said.

Claytor, currently a lawyer with the Washington firm of Covington and Burling, said he has ridden Amtrak a "good deal" and present "Amtrak service is so good that it's not even in the same league" as the system Boyd inherited four years ago.

Claytor said his twin goals are to improve service and keep costs under control. He declined to be specific about his plans for Amtrak, saying he would decide later whether the system should be "larger or smaller" after studying the "pros and cons." Important considerations are cost-effectiveness, need and potential traffic, he said.

Claytor, who was with Southern from 1963 until 1977, held a number of high government posts in the Carter administration, including deputy secretary of Defense, secretary of the Navy and, briefly, acting Transportation secretary. He joined Southern as vice president of law, became president in 1967 and chairman in 1976.

Asked about Claytor's Democratic political ties, Transportation Secretary Drew Lewis, who served on the Amtrak board's three-person search committee, called Clayor the best qualified of the 20-plus candidates, quipping that "there was no Reagan campaign coordinator who was qualified" for the job.

Lewis said the Reagan administration is committed to keeping a national passenger rail system that he envisioned fundamentally as "a corridor system" in highly populated areas, with several East-West and North-South long-distance lines in addition.

Lewis said he will continue to oppose the "politicizing" of Amtrak. When Congress "gets involved" and a legislator insists on a train through his district "and there is no justification to it whatsoever, I'm in opposition," he said.