When Jimmy Carter was president, Washington's submway planners looked confidently to the opening in 1985 of Metro's Orange Line extension to Vienna, to the opening in 1987 of the inner-city Green Line from Anacostia to U Street and to the completion by 1990 of the 101-mile system. a

In Baltimore, planners looked confidently to the opening late in 1982 or early in 1983 of the first eight miles of their subway system and to federal financial support for six more miles. They hoped eventually to win federal approval for a 71-mile system.

Now that Ronald Reagan is president, Metro officials estimate, Washingtonians will have to wait at least a year longer for both the Vienna extension and the inner city Green Line, and it is possible the Metro system will be cut from 101 miles to fewer than 80.

And in Baltimore, transit officials are certain only that eight miles of subway will be completed. A six-mile extension, the Reagan administration says, is worthy, but the federal government cannot help build it until the economy improves, whenever that is. The 71-mile system seems more remote than ever.

Those are just two examples of how the Reagan administration is proposing to slow or stop subway construction around the country by reducing or withholding federal dollars as it cuts domestric spending.

"The construction of new rail transit systems and extensions of existing systems has not proved . . . to be as cost-effective as less capital intensive projects," according to the Office of Management and Budget's explanation of the Reagan budget revisions for 1982.

If Congress accepts the Reagan budget proposals, the nationwide kitty for building subways and buying buses in 1982 will be cut $409 million from the $2.9 billion Carter proposed. From 1982 through 1985, according to administration projections, a total of $4 billion less would be spent to buy both buses and subways than Carter projected.

Further, Carter budgets assumed that about one-fourth of federal transit investment would be spent to build or expand rail systems. The Reagan administration assumes no new systems will be built and few extensions approved. Most transit money would go for buses or to modernize old subways such as New York's and Philadelphia's.

That means Baltimore and Washington would have to compete for fewer dollars with Atlanta, Buffalo, Portland and Miami, all of which have rail systems under construction or promised.

"Proposed extensions to these cities systems would almost certainly not be funded, . . ." according to a Congressional Budget Office analysis. Key Department of Transportation officials, from Secretary Drew Lewis down, have ducked the question of how they would define an extension.

There would be no money for Detroit, Honolulu, Houston and Los Angeles to start their rail transit systems. All four are well along in planning.

The biggest problem with subways is that they are incredibly expensive; thus they make good targets for federal cost-cutters. Washington Metro, for example, in 1968 had a projected final cost of $2.5 billion; the current official estimate after 13 years of inflation and delay, is $8.2 billion; there are few at Metro who think it can be done for less than $10 billion.

Subway fans argue, however, that the investment in tunnels and track is one made for a century or more and cannot honestly be equated with the purchase price of a new bus fleet.

Secondly, once construction has started on systems such as Washington's and Baltimore's, cutting back what was planned often means an inefficient operation. For example, a 14-mile line in Baltimore would cost only 25 percent more than the eight-mile starter line and would increase ridership by 27 percent. Baltimore

When it is finished, the Charles Center subway station will be written about as one of the engineering and transportation marvels of this age.

It runs for almost three blocks under East Baltimore Street; its $41 million price tag includes an $8 million modification to make sure the 26-story Maryland National Bank building does not fall into the tunnel, and it is big enough to handle more than 40,000 passengers a day.

That is the dream. The reality is that it will be years before Charles Center approaches 40,000 passengers a day, if ever. For the foreseeable future, Charles Center will be nothing more than the southern terminus of the first eight miles of Baltimore's subway and one of nine stations now projected to open either late in 1982 or early in 1983.

The huge station may never be the bub of a five-spoke, 71-mile subway system Baltimore visionaries have been dreaming about since the early 1960s. Baltimore's immediate worry is not for 71 miles, but for 14: the eight-mile starter line plus its threatened six-mile extension.

L.A. (Kim) Kimball, Maryland's mass transit administrator, said deferring the extension would mean "more of a schedule change more than a direct impact. . . . We could hold over for a year or so." After that, Kimball said, the price of dealy would climb sharply.

Baltimore could not exercise costsaving options on subway cars, electrical systems and signaling device, and key members of the construction management team would have to be dismissed, their experience probably lost forever.

The eight-mile starter line will run from the Inner Harbor area (at Charles Center), through the famous Lexington Market, the state office building complex and a decaying inner-city residential area before terminating in the back lot of Reisterstown Plaza, a shopping center. Planners predict that 83,000 people a day will ride the subway on those eight miles, which cost U.S. and Maryland taxpayers $768 million.

The six miles and three stations in the extensioin are estimated to cost another $190 million and bring 22,000 more riders to the system.

Without the extension, the subway will end about three miles short of the Baltimore beltway, a route planners feel it must intersect if it is to make a serious impact on auto traffic on crowded Reisterstown Road.

"The beltway argument is a red herring," said Arthur Teele, the U.s. Urban Mass Transportation Administration chief. ". . . They didn't even project operation [of the extension] before 1985. All we're asking them to do is stretch it out a little bit more."

For how long?

"I would like to reserve the right to decide," Teele said. "It will be when the economy improves. The staff feeling is that the extension should be built." Washington's Metro is now a proven system that is beginning to do all the things subways are supposed to do. It is carrying hunmdreds of thousands of people, attracting developers to both the center city and suburban downtowns and providing easy daytime trips across Washington's sprawling metropolitan area.

It was supposed to do more, but because of the Reagan administration's generally dim view toward subway extensions, there is a substantial question as to whether all of the planned 101 miles will be completed. There is no question that Metro's schedule for completing the subway will be delayed.

The Carter administration had told Metro to plan an annual construction budget of $350 million in federal aid beginning in 1982. If that rate of federal expenditure lasted for several years, the 101-mile system would be completed early in the 1990s.

It never happened. The Reagan administration has proposed cutting Metro to $275 million in 1982, but is unofficially projecting $375 million in 1983 and possibly $325 million in 1984. That works out to $75 million fewer federal dollars over three years than Carter had proposed.

There apparently are no federal projections for Metro beyond 1984. That may be meaningless but it could also indicate that administration officials are considering a cut in the Metro system's mileage.

There is also the possibility that, in 1982, the District of Columbia will claim $54 million of that $275 million for D.C. highway projects.That is the District's right, but the effect would be to reduce federal money available for Metro to as little as $221 million.

It would be difficult, Metro officials agree, for local governments to work out a construction agreement with so little money available, because such agreements must appear to provide something for everybody if they are to pass local boards and councils.

Another source of funds is available for Metro -- specifically the $1.7 billion Stark-Harris authorization Congress passed in 1979, but the administration has yet to decide whether to use it, according to Teele, the federal urban mass transportation administrator. Early federal projections show some Stark-Harris money in 1983 and 1982, when the money could be used to offset whatever the District needs for highways.

D.C. federal and Metro officials are just beginning to discuss the implications those facts hold for the construction schedule.

Metro is already assured of completing 61 miles, the 37 now in operation and:

The Red Line from Dupont Circle 16 miles north to Shady Grove Road in Montgomery County. The first two miles and three stations -- Woodley Park-Zoo, Cleveland Park and Van Ness-UDC, will probably open this December. The remainder would open late in 1983.

The Blue Line from National Airport five miles south to Huntington in Fairfax County, to open late in 1982.

The Yellow Line, a three-mile shuttle across the 14th Street Metro Bridge from the Pentagon to Gallery Place, to open late in 1982.

The Metro boardhas not agreed on what to build beyond those openings, although work is in progress on several sections. The best guess is that local agreements will be reached that permit:

The opening of the Orange Line extension from Ballston 9.12 miles west to Vienna. Under the Carter budget projections, it would have been possible to open that line sometime in 1985. Under the Reagan projections, mid-1986 is the earliest possible date, Metro General Manager Richard S. Page estimated.

The opening of a five-mile, inner-city Green Line section from Anacostia to U Street NW at 14th Street. Metro officials have estimated they could open that in 1987. Page is now estimating the opening date as early 1988.

That would give Metro 75 miles. It is impossible at this time to predict the future of the remaining 26 miles, although substantial preliminary design and construction work has been done on the first two segments in the following list:

The Red Line from Silver Spring north to Glenmont.

The Yellow Line from Alexandria's King Street station 6.14 miles west to Franconia-Springfield in Fairfax County.

Two legs of the Green Line. The southern leg would run south from Anacostia to Rosecroft Raceway in Prince George's County; the northern leg would run northwest from 14th and U Streets NW into Prince George's County and terminate in Greenbelt.

Just as is the case in extending Baltimore's system, administration officials such as Lewis speak warmly of Metro beyond 75 miles, but make no commitments.