Outside the densely populated Boston-Washington corridor, the nation would save energy if all rail passenger services were halted, according to a new study by the Congressional Budget Office.

However, the savings would be small. If Amtrak operated only along the Northeast Corridor, the fuel savings would equal less than one-hundredth of a percent of national petroleum consumption. Driving every car five fewer miles each year would save more.

The CBO study dashes cold water on one of the major claims made in recent years by supporters of Amtrak - that switching people from cars and other carriers would save petroleum.

Both the current Amtrak system and a sharply curtailed version favoured by the Carter administration "lose energy since the savings in the Northeast Corridor are more than offset by losses outside the corridor," said the CBO staff, in a study made available by Rep. Robert Duncan (D-Ore.).

But the additional finding, that cutting out passenger trains would save very little energy, is certain to be cited by Amtrak supporters in their arguments that, with another petroleum shortage now possible, this is not the time to start eliminating trains that could become more energy-efficient if there are more passengers.

A train carrying hundreds of people is more efficient than private automobiles but relatively few people use trains and there is not adequate rail equipment to handle a huge increase in riders.

The National Association of Railroad Passengers says that "adequate" development of a national intercity rail service over the next 15 to 20 years could save 3.7 percent of U.S. petroleum consumption.

The findings by CBO - which differ with other studies of rail energy efficiency - are being debated on Capitol Hill. Unless either the Senate or House acts by May 23 to veto administration plans that would eliminate service on 43 percent of Amtrak's route miles, the restructuring takes effect Oct. 1.

Congressional staff members said yesterday that no such veto motions are anticipated, although a House sub-committee has approved new "route criteria" that would effectively amend the administration's plan by retaining three or four trains now scheduled for the end of the line.

Rep. James Florio (D-N. J.,) chairman of the transportation subcommitee, has said the Montrealer between the United States andMontreal has the best chance of being given a new life, provided the Senate accepts any House action. Florio's subcommittee supports an additional$35 million on top of the administration's request for Amtrak funding of$552 million in fiscal 1980, to run the extra trains.

Established by the government in 1971 to preserve a skeletal nationwide rail passenger network, Amtrak (its formal name is the National Railroad Passenger Corp.) operated with a deficit of$582 million last year. Although ridership has increased 6 percent in recent months, Amrtak accounts for less than one percent of intercity travel.

Secretary of Transportation Brock Adams has called for saving $1.4 billion over the next five years by halting service over 43 percent ofAmtrak's 27,000-mile network.

Among the study's findings:

A signicantfactor in energy efficiency along the Boston-Washington corridor is electrification, which now powers most of the route and is being expanded at a cost of $2.5 billion to Boston.

If a petroleum shortage develops, trains in other dense corridors would be the most energy-efficient way tomove people. Duncan, who requested the study, said it "appears to indicate that energy savings from Amtrak have been blown out of proportion . . . The evidence to date suggests that buses are a more energy-efficient and less expensive mode of intercity transportation, and that resources now allocated to Amtrak could save more money if reallocated to urban mass transit or to develop a more energy efficient automobile."