The Interstate Commerce Commission gave temporary approval yesterday for an affiliate of Greyhound Lines to operate Trailways Lines Inc.

The 5-0 ICC vote approving the joint operation of the nation's two largest bus companies took place the same day the Justice Department advised the commission that Trailways qualified as a "failing company" under federal antitrust laws and that therefore the proposed acquisition by Greyhound of Trailways would not violate those laws.

Justice Department attorneys, economists and financial analysts found Trailways "on the brink of bankruptcy" with "virtually no chance of being restructured as a viable entity" and there are no other potential purchasers for the firm, the department's antitrust division said in a filing with the ICC.

Under the temporary approval granted by the ICC, GLI Acquisition Co., which controls the Greyhound bus system, will operate Trailways Lines Inc. and plans will go forward for the $80 million acquisition of Trailways, a transaction Greyhound publicly proposed June 19.

The ICC eventually will decide whether the merger is to be made permanent. Therefore, the transaction under the commission's order could be undone.

The two bus lines will begin operating as one next week. "I think the transaction is really good news for users of low-cost transportation," said Fred G. Currey, Greyhound's chief executive officer.

Asked about pricing under the new structure, Currey said that "we have reduced prices in thousands" of routes between two cities "in the past 90 days where we are sole provider.

"This industry and our company has no pricing power. We price, not against buses, but against automobiles and discount air fares, and business has been based on that policy since the day we bought Greyhound Lines," he said.

Currey and two other Texas businessmen, Craig Lentzsch, vice chairman and executive vice president, and Anthony Lannie, executive vice president, purchased Greyhound March 18 and turned it into a privately held company.

Greyhound and the privately held Trailways are both Dallas-based companies.