House Transportation appropriations subcommittee members sharply questioned federal officials yesterday about why federal support for Metro should continue in the absence of a plan binding local governments to pay for Metro's operating and construction costs.

Rep. Robert B. Duncan (D-Ore.), subcommittee chairman, noted that "virginia has once again voted down a tax source for Metro. There is no assurance that the Maryland issue will be resolved, and the local governments want to complate the (planned) 100-mile system and expect that the federal government will continue to bail them out."

Mortimer L. Downey, assistant federal transportation secretary and a key player in recent federal-local negotiations on Metro, said that Washington area governments have made "a major step forward" in the past year in drafting a financial plan that sets forts "for the first time honest and complete" estimates of what Metro will cost.

That estimate is a $7 billion total construction cost, assuming completion by 1987. Not included are the repayment of interest and principal on $1 billion in revenue bonds sold to build Metro or the financing of annual operating deficits that now exceed $100 million annually and are projected to run as high as $500 million annually by 1990.

Downey was testifying at a hearing on the proposed Carter administration budget for the Urban Mass Transportation Administration (umta). umta c/hief Richard S. Page disqualified himself from answering Metro questions because he is the leading candidate to replace Theodore C. Lutz as Metro general manager.

The federal budget proposal includes $275 million each year in fiscal 1979, 1980 and 1981 for Metro construction. That is $125 million less than local officials say they need to remain on schedule.

Metro, however, is not alone. During two days of hearings on the UMTA budget, one transit expert after another has managed to lament through one congressman or another that there is not enough money in the budget to finance all the various subway, elevated and trolley construction programs currently under way nationally.

The American Public Transit Association has been meeting in Washington this week so its members could lobby congressmen for increased capital and operating assistance. New York City, for example, discovered that the president's budget contained $28 million less in operating assistance for its huge transit system than was authorized by the last Congress.

while congressmen appear sympathetic, they have other problems. Subcommittee chairman Duncan said in an interview yesterday, "We're all painfully aware of the pressures for a constitutional convention" that would require the federal government to balance its budget.

As far as Metro itself is concerned, Duncan said, "It's very difficult once you start something to truncate it. On the other hand I do not want the taxpayers of Minneapolis, Portland and Miami to be forced to carry Metro on their backs."

Metro's present financial structure, which depends on voluntary contributions from seven local governments to finance the operating deficit, is "untenable," Duncan said, because "one unit of government can frustrate the whole system." On the other hand, he said, "we do not want to dicate the solution."

The recent defeat by a Virginia House committee of a Northern Virginia proposal that would permit a dedicated sales tax for Metro deficits "was a setback," Downey testified. Transportation Secretary Brock Adams will visit Annapolis today to urge the Maryland General Assembly to adopt some kind of tax aid for Metro in Maryland.

Duncan also grilled Downey about an internal Department of Transportation memorandum last July that charged, among other things, that there were serious defects in the regional restudy of Metro's uncompleted lines. As a result of that federally ordered restudy, local officials voted to complete the 100-mile system essentially as originally planned in the late 1960s.

A central point in the memorandum was a charge that the restudy grossly overestimated Metro's potential ridership. The study projected rail ridership at 323 million trips annually by 1990 -- about three times the present level.

Downey said that Metro had revised those estimates downward and was undertaking, at federal direction a new, detailed analysis of ridership. The restudy of Metro was not intended, its authors said many times and Downey reiterated yesterday, to be a definitive ridership survery. Rather, it was designed to show relative values of one kind of transit or transportation versus another in a given corridor.

"Would it be helpful," Duncan asked Downey," if Congress took additional action" to prod local government to complete their financial arrangements.

"To the degree that it doesn't remove our options," Downey replied.