The administration asked Congress yesterday for an additional $750 million to upgrade the passenger rail corridor between Washington and Boston.

The proposal, which would raise total funding for moderniation of train travel on the Northeast corridor to $2.5 billion, would also extend the completion date of the project three years to 1984.

The goal of the project,which was authorized by Congress in 1976, is to achieve dependable Amtrak service along the 456-mile corridor at speeds of up to 120 miles an hour. If trip goals are achieved, passengers would be able to travel between Washington and New York in two hours and 40 minutes and between New York and Boston in three hours and 40 minutes.

Federal Railroad Administrator John M. Sullivan blamed the need for the additional funds on the "cloudy crystal ball" of the original planners who didn't anticipate the high inflation rates of recent years. But he insisted the commitment to the original idea was even more important today.

"By sticking to the original basic project, we can produce an electrified, high-speed public transporation system, potentially independent of the need for petroleum," he said at a news conference. Noting that the northeast rail service will serve 23 million people, measurably reducing their dependence on cars and the gasoline they consume, he added, "this benefit . . . cannot be overemphasized in view of our dependence on foreign petroleum and the threat that imposes on our national security and economic well-being."

Louis S. Thompson, director of the Northeast Corridor Project for DOT, defended the time extension the admnistration was seeking on grounds that the work schedules for the project were "over-optimistic to begin with -- especially if you try to keep the railroad operating while you do the construction."

The project involves electrifying the corridor and overhauling and rebuilding tracks, bridges, crossings and stations.

Rep. James F. Florio (D.-N.J.), chairman of the House subcommittee on transportation and commerce, pledged at the news conference to hold hearings on the administration proposal early next year and to enact legislation in the spring or by early summer at the latest.

"We must sell this to Congress not as a regional project but as a model for the nation," he said. "The lessions we learn rom the Northeast Corridor can be applied to other densely poopulated areas," He noted that other potential candidates for similar rail service that have been identified by the Transportation Department include the triangle between Dallas, Houston and San Antonio; the corridor between San Diego and Los Angeles; and the Buffalo, N.Y., area.

"I'm convinced people will go back to rail if it's high quality," Florio said.

Project director Thompson said the benefits of available mass transportation are shown clearly by the gasoline consumption figures for the first six months of this year. In the Northeast corridor area, gasoline consumption was 23 percent less per capita than around the nation during the first half of the year because mass transporation is available there, he said. Saving gas in the Northeast means it can be used elsewhere where no mass transportation is available, he noted.