If rail passenger service is to survive in the United States, Amtrak must be given an opportunity to retrench and rebuild, Amtrak President Alan S. Boyd told a Senate subcommittee yesterday

"We've got to have a smailer system if we're going to have a system at all," Boyd said. "We're having breakdowns; our maintenance costs are out of sight and there's a lot of junk we're hauling around out there." The worst thing that could happen to Amtrak, Boyd said, "is to leave us with the system we have today."

Boyd's testimony came on the first day of hearings on whether Congress will accept a Department of Transportation plan that would cut Amtrak's route miles by 43 percent and realign several routes. If the plan is not rejected by either house of Congress by May 2, it will automatically go into effect in October.

The plan was vigorously defended against attacking senators yesterday by Transportation Secretary Brock Adams, whose department created it. "We went through those routes (that Amtrak presently serves) inch by inch" in deciding which routes to cut, Adams said. If a state wants to restore some of that service, then the state should be prepared to pick up part of the subsidy that is required to operate Amtrak, he said.

Adams appeared to gain valuable support from Sen. Russell Long (D-La.), chairman of the Surface Transportation Subcommittee of the Senate Commerce Committee. But while Long said "we're going to have to operate a modern system or we better not operate at all," he also mourned the passing of the Southern Crescent, the train from Washington to New Orleans that is eliminated in the Adams plan.

Adams contends that the proposed route restructing will save $1.4 billion in subsidies over the next four years, while continuing to provide rail service to 91 percent of those now using the system.

He disagreed with suggestions from several senators that Amtrak was needed in times of uncertain energy availability by saying that "if all of Amtrak's equipment were presently operating on the existing system, it could carry only 1 percent of the intercity trips" made by travelers in the United States.

Sen. Harrison H. Schmitt (R-N.M.), whose state would lose Amtrak service to Albuquerque, charged that Amtrak had not adequately tried to sell the service it has and that the new route structure lacked feeders to build up patronage on main lines.

"The feeder lines are going to have to be bus and car," Adams said.

Sen. Patrick J. Leahy (D-Vt.) complained about the proposed elimination of the Montrealer, a train that runs in his state; Sen. Warren G. Magnuson (D-Wash.) noted that four of six trains to Settle had been eliminated, and so it went.

Nonetheless, Adams' people feel that the route realignment will survive the congressional test. "I think we will be able to persuade both houses to go along with us," said Terrence L. Bracy, assistant secretary for governmental affairs. "We're visiting every member of the House and the Senate and explaining the costs involved."

Boyd, who said he was willing to operate whatever system Congress dictates, quibbled with some specific details of the Adams plan, but not with the general outline. He said Amtrak could operate within the proposed federal subsidy of $550 million for fiscal 1980, but that guarantees beyond that were difficult.