Arthur D. Lewis, chairman of the U.S. Railway Association and a former airline executive, has been selected to become the bus industry's top spokesman.

Informed sources said yesterday that Lewis will succeed Charles Webb, president of the National Association of Motor Bus Owners, who announced last year his plans to retire. NAMBO's board members are gathering here today for regular meetings, during which the Lewis selection may be announced.

The choice of Lewis comes at a time when the bus industry is seeking federal aid to maintain and buy new buses and other facilities. In addition, the industry is escalating its attack on Amtrack, the government-subsidized national rail passenger corporation.

Last week, a bus industry attack on Amtrack's fare structure, won conditional approval from the Transportation Association of America, a diverse organization that represents most sectors of the country's transportation industry.

The TAA plan - which still must be ratified of rejected by the Association of American Railroads, of which Amtracl is a member - would promote legislation to require Interstate Commerce Commission approval of the rail corporation's fares.

In effect, under ICC rules, fares that don't cover costs would be ruled illegal. Since Amtrack operates with an annual loss ($438 million for the year ended last Sept. 30), the TAA argues that federal taxpayers are picking up $24 of the ticket cost for every Amtrak rider.

A TAA study concluded that it is unfair for bus firms to provide a service and invest capital when part of the competition is subsidized, "when Amtrak plays by a different set of rules that may result in private enterprises being out of business by a government enterprises."

Lewis, who sucessfully engineered through Congress a plan to reorganize bankrupt railroads in the Midwest and northeastern states, was named to head the U.S. Railway Association in 1974.

Previously, he was chairman of a New York investment firm, president of both Eastern and Hawaiian airlines and a top officier of American Airlines. Ironically, Lewis also was one of the original "incorporators" of Amtrak, founded in 1971 to operate a national network of passenger trains at a time when private industry was seeking to end most remaining services.

Through a spokesman, Lewis said yesterday that he sees the bus job as "a new challenges to prominently position the bus industry in the ranks of the nation's transportation modes."

Speculation is that the rail association board will meet June 30 and consider William K. Smith to be president for transportation at General Mills, Inc., in Minneapolis. The chairman must be appointed by the President and confirmed by the Senate and sources said they except Smith to be picked.

The rail association, a planning and financing agency, has as its main task in future years the monitoring of Consolidated Rail CorP., a new company formed to succeed the bankrupt rail operations of Penn Central and half a dozen other irms.

In recent congressional testimony, the bus owners denounced Amtrak's competition and called for what would amount to the first federal financial assistance for bus companies.

Althoughthe bus companies did not attach any dollar amount to their initial request, they asked for subsidies to cover losses on some intercity operations, grants for terminal construction and an eased tax burden - including a proposed $100 million refundable tax credit.

Bus traffic and profits have been declining in recent years and leaders of Greyhound Lines and Trailways (a Holiday Inns subsidiary) have blamed part of their problems on competition from Amtrak in key markets - including the Boston-Washington corridor.

The Association of American Railways boards is scheduled to meet June 24, at which the new anti-Amtrak transportation association resolution will be discussed. The board, which includes Amtrak president Paul Reistrup as an ex-officio member, is thought to be divided on whether the industry should support the bus association move.

Legislation that established Amtrak prohibited the ICC from acting on the passenger company's fares. If Amtrak is forced to apply total csots to each of its train, fares could be expected to rise sharply - especially on money losing long distance routes.