A stopgap plan for continued financing of Metro construction, including the completion and operation of three routes into suburban Maryland, has been revived after being sidetracked by a legal challenge in Virginia.

If, as Metro officials now predict, the seven otaher affected local governments in the region ratify the financing plan, work can be extended without interruption on six rail routes totaling 60 miles from downtown Washington into the Maryland and Virginia suburbs.

The dispute had threatened to halt the award of further contracts on the rail system.

The impending and almost-certain agreement on stopgap financing is one of several recent actions that are expected tro affect the extent and shape of the projected 100-mile rail system.

Although the Fairfax City lawsuit attacking Metro's stopgap financing plan is still alive and scheduled for a hearing Feb. 1, the Metro board voted to eliminate a clause from the plan that formed a major basis of the city's complaint. The clause was interpreted as meaning that only a 60-mile rail system would ever be built, and that the Vienna line, which would serve Fairfax City, would be eliminated.

Following Metro's action, the Fairfax County Board of Supervisors voted unanimously Jan. 10 to approve the stopgap plan. After a meeting two days later with transit officials, Mayor Harold L. Miller of Falls Church - the last holdout - predicted that the City Council will approve the plan next Monday.

That would permit Metro to ask the U.S. Department of Transportation to begin releasing funds to continue work.

The stopgap plan is intended to stretch available and potential funds - a total of $942.8 million - to finish building and start operating the 60 miles of routes while the fate of the officially adopted 100-mile system is being determined politically.

The 60 miles would include routes from downtown to Shady Grove, Silver Spring, New Carrollton and Addison Road stations in Maryland and the Huntington and Glebe Road stationship Virginia.

Structual work on all or part of these lines already is under way. The new funding would permit the finishing of stations and the installation of rails and other equipment to permit the routes to begin operation.

Much of the other unbuilt mileage is being subjected to a study, mandated by Congress and the U.S. Transportation Department, to see if cheaper forms of transit could be substituted.