Although completion of Metro's 101-mile system still is a decade away, area congressmen yesterday began pushing for an even larger subway.

Rep. Herbert E. Harris II (D-Va.), joined by three other area legislators, introduced legislation that would call on the federal government to underwrite the costs of studying the addition of another 47 miles to Metro.

Their proposal calls for four possible extensions to the subway:

A 10-mile link to Burke in southern Fairfax County that would connect with the planned Yellow Line near the Capital Beltway and Van Dorn Street just south of Alexandria.

An 18-mile link along the Dulles Access Road that would serve Dulles International Airport, Reston and Tysons Corner and tie in with the Orange Line at the planned West Falls Church Station.

A six-mile extension of the Red Line from its planned terminal at Shady Grove Road in Montgomery County north to Germantown.

A 13-mile extension of the Blue Line from the Addision Road terminal in Prince George's County, past the Capital Centre in Largo and then northeast to Bowie.

The study should also consider possible expansion within the District of Columbia, perhaps along H Street NE and NW, Harris said.

Two of those proposed lines -- the Orange Line spur to Dulles Airport and the extension of the Blue Line to as for the Capital Centre, have been considered logical entensions to Metro for some time.

The Burke and Germantown extensions, along with others not mentoned by Harris, have been on Metro's unofficial list of possible extensions for years.

However, because of difficulties completing the 101-mile system have been so great, few political leaders have talked seriously about extensions.

Harris insisted that the proposed study was realistic. Another sponsor mustn't confuse a study with a conclusion that all of these dotted lines will be built."

Harris, who was the prime mover behind legislation that authorized what was supposed to be enough federal funding to guarantee completion of the 101-mile system, said that "I think we have a tremendous advantage right now for legislation like this" to pass.

Rep. Michael D. Barnes (D-Md.) added that "I think this is going to look less and less like pie in the sky as gasoline hits $2 or $2.50 per gallon. lThere's going to be increased impetus for more Metro."

The total cost of the study would be $1.8 million, with $300,000 to come from local sources. Harris said he hoped Metro -- not a consultant -- would do the study. "But I take the position," he said, that "if in the short term it can be done better by a consultant," then so be it.Harris has been flaying consultants recently in congressional hearings.

While the congressmen and D.C. Del. Walter Fauntroy were talking of more subway tracks, Metro staff members were putting the final touches on a proposal they hope will resolve the subway's most recent financial crisis.

Funding for Metro's Fiscal 1981 construction program has been delayed for months while Maryland sought legally binding guarantees that Virginia and the District of Columbia would continue to contribute to the cost of building uncompleted Maryland sections of the 101-mile system after the Virginia and D.C. sections were completed.

A compromise that will provide 15 months for working out those assurances was agreed to yesterday by Maryland, Virginia and D.C. officials and is expected to be approved by the full Metro board today.