The Reagan administration's proposed fiscal 1982 funding level for Amtrak operations probably will doom all its rail passenger service except in the Northeast corridor, Amtrak President Alan S. Boyd told Congress yesterday.

"At the $613 million level, we calculate that Amtrak would be able to provide service only in the Northeast Corridor, between Washington and Boston, beginning in October of 1981, at the start of the new fiscal year," Boyd said in a letter to Rep. James J. Florio (D-N.J.), chairman of the House transportation subcommittee.

The administration's $613 million figure represents a reduction of $380 million from the budget authorizations sent Congress in January.

In Boyd's letter, which added details to his testimony yesterday before the subcommittee, he said it was unnecessary to detail the impact of the budget cuts route-by-route because the proposed budget "leaves little flexibility" for Amtrak operation.

Boyd said Amtrak has an "irreducible capital commitment" of $130 million for equipment purchases; $250 million in "infrastructure costs" in operating the Northeast Corridor, not including $82 million in interest costs that he assumed would be lifted; $10 million in other Northeast Corridor costs that are not offset by revenues; and $200 million in labor protection costs mandated by various laws. That leaves $25 million for "shutting down the rest of the system," Boyd said.

The administratioln's request for budget authorization of $613 million for Amtrak includes money for both operating costs and capital needs. Amtrak had sought $716 million for operating costs alone and had counted on several hundred million dollars more for already-authorized capital expenditures.

Robert E. Gallamore, deputy administrator of the Federal Railroad Administration, told the subcommittee that the administration was preparing to send to Congress proposed legislation to amend Amtrak's authorization in keeping with its revised budget submissions.

Testifying in place of Robert Blanchette, who was awaiting Senate confirmation as ERA administrator, Galamore defended proposed cuts in the budget and in Amtrak's route system, although he said the administration had not drawn a list of specific trains to cut.

"We recognize the need for the Northeast Corridor and possibly other segments of the Amtrak system which have similar short-haul, high-denisty characteristics," he said. "Apart from that, however, we find little in terms of economics, energy conservation or other public benefits to justify continuation of Amtrak operations within its present context."

Florio complained about the Transporation Department's testimony on Amtrak, agreeing with Boyd that the proposed budget cuts would result in the "virtual dismantling" of the national rail passenger system with the exception of the Northeast Corridor.

"It's irresponsible for the administration to announce a proposed budget cut of about $400 million for Amtrak and then not bring a single proposal to Congress on how to restructure the present system," he said after the meeting.