The Metro subway system, a multibillion-dollar monument to urban mobility and fiscal controversy, will mark its 10th anniversary today with many of its originators' dreams still hanging in the balance.

Touted as an engineering triumph and architectural showpiece, the Washington area's $5 billion, 60.5-mile rail network has changed the commuting and leisure-time habits of thousands of city and suburban residents. But many other would-be riders have been left beyond the subway's reach.

The system, once expected to be completed by the late 1970s or early 1980s, remains more than 30 miles and $2 billion short of its goal. The Green Line, designed to serve many of Washington's poorest residents, has not opened. After court battles and controversy, the line's fate hinges on a new fiscal crisis.

The rail system, hailed by its advocates as "America's Subway," has drawn lavish tributes and pointed criticism since it opened as a mere 4.6-mile shuttle on March 27, 1976. The initial Red Line section linked the District's Farragut North and Rhode Island Avenue stations.

As it expanded, the subway helped shape broad regional trends and countless day-to-day decisions. People chose where to live, whether to buy or sell a car, where to eat lunch or dinner and sometimes whether to take a new job because of prospects of traveling by Metro.

Commuters packed rush hour trains to avoid traffic backups on snowy winter days. Families took the subway on weekend outings. For some elderly residents, the rail system meant new-found mobility. Riders grumbled about high fares and malfunctions, but praised the system's speed and convenience.

"This subway system is so much better than any you can think of," said Joseph S. Wholey, a university professor who once headed the Metro board. But he added, "It could be a little bit more pleasant if I could get on the train instead of being left on the platform." Crowding is a recurrent complaint.

General Manager Carmen E. Turner calls Metro "the miracle" -- a vast project carried out in the face of financial obstacles and political conflicts. The subway, she argues, is popular, clean, reliable and safe.

"What we have seen over the past 10 years is a maturing system. We have learned a lot," Turner said. "Can it be better? Of course. But that's the real challenge."

Ralph L. Stanley, the Reagan administration's mass transit chief and one of Metro's most outspoken critics, denounces the system's policy-setting board as "fiscally irresponsible." He points to longstanding cost overruns and prospects of skyrocketing expenses to replace worn-out rail cars, tracks and equipment.

"Metro has failed to confront the true cost of the system," Stanley asserted. "There is so much unfunded liability."

The price of finishing the proposed 103-mile system, initially estimated at $2.5 billion, has soared to $9.4 billion, according to Metro's calculations. The target for winding up construction has slipped to 1997. Costs could go higher, officials say, if work is delayed beyond then.

The Reagan administration, citing these mammoth overruns along with the nation's deficits, has threatened to cut off spending for Metro construction, leaving the transit authority's elaborately charted plans a shambles.

As a fixture in the urban landscape, the subway has set off a far-reaching debate among transportation analysts, planners and other government officials, who have sought to measure its impact and gauge why the system has, in some key respects, fallen short of predictions formulated in the 1960s and 1970s.

The number of subway passengers -- about 400,000 on an average weekday -- has hit new peaks and helped raise overall mass transit patronage to the highest level since the end of the Korean War when Washington was served by privately run bus and streetcar companies. Yet ridership lags far behind Metro's early forecasts.

The subway system has helped to ease rush hour traffic congestion in the downtown area. Nonetheless, traffic on city streets has steadily increased in the last few years, and highway bottlenecks have severely strained many suburban communities that the subway does not serve.

"Metro is not a panacea. It will never replace the automobile," said William I. Herman, Metro's planning director.

More than $3 billion in commercial development has spread through areas near Metro stations, according to recent studies. But this development has stirred mixed views among regional analysts, with some terming it less than expected. Jobs have increased relatively slowly in areas near the stations.

The Metro system, jolted by a derailment that killed three passengers in 1982, has taken major steps to improve safety. After widespread complaints about train breakdowns and equipment malfunctions several years ago, Metro officials have engineered a turnabout, showing marked gains in reliability.

At the same time, government officials and commuters have expressed mounting irritation about inadequate Metrobus connections to the subway and shortages of parking spaces at Metro stations.

The rail system's failure to fulfill some of its early promise stems, in large measure, from social and economic shifts that were not foreseen by the subway's planners, transit analysts now say. Inflation was higher, population growth slower and suburban sprawl more widespread than had been expected.

When the subway was planned, the area's population was predicted to reach 4.3 million by 1990 -- which is 1 million more than now considered likely. Growth in jobs and populace was expected to be concentrated near the region's center. Instead, it has occurred mainly in outlying suburban areas.

"That's what hurt us," said Robert A. Pickett, Metro's assistant planning director. "Our market is at the core. It's in the very areas where we do the best that the growth is stagnating or slowing down very substantially."

The automobile also proved a more formidable challenge for Metro than expected. Car ownership rose by nearly 40 percent from 1960 to 1980, as two-earner families led increasingly to two-car households. Metro's proponents overestimated the convenience of mass transit, some officials say.

Ronald G. Sarros, a veteran transportation analyst for the Metropolitan Washington Council of Governments, recently urged regional officials to adopt a "more realistic estimate" of mass transit's impact.

Not long ago, planners had expected 55 to 60 percent of the commuters heading into the downtown area to travel by subway or bus, Sarros pointed out. Instead, transit riders account for only 33 percent. Even by 1990, Sarros said, the level may barely exceed 40 percent, markedly less than forecast.

Hamstrung by these trends along with delays in subway construction, the rail system is yet to approach its long-predicted target of 650,000 passengers a day. Even if forecasts are scaled back to accord with today's unfinished 60.5-mile system, officials say, ridership still lags by about 25 percent.

Some of the system's recent improvements were outgrowths of earlier mishaps. Chief among these was an overhaul of safety procedures triggered by the 1982 derailment.

The fatal accident, which occurred just a half-hour after an Air Florida jet had crashed into the 14th Street bridge, led to several investigations, recommendations and changes in Metro's operating rules, training and equipment.

"My personal opinion of the operation is that it's considerably improved, more professional," said John Rehor, who headed a National Transportation Safety Board investigation of the derailment. All but four of the panel's 34 recommendations have been carried out by Metro, officials said.

Nonetheless, some safety officials have expressed continuing concern. Metro's refusal to institute a proposal to restrict speeds of slow-moving trains leaves open the possibility of future collisions, Rehor warned. Costly improvements of Metro's control center also are needed, Metro officials say.

Since 1982, no derailment, collision or similar accident has occurred during regular service, according to Metro officials. Fady P. Bassily, Metro's assistant general manager for rail service, argues that adequate "checks and balances" have been established to make such accidents unlikely.

"How much is enough?" Bassily asked. "You still have a human element here."

With a Northern Virginia extension of the Orange Line scheduled to open to Vienna in June, the system soon will encompass nearly 70 miles and 61 stations. Construction is under way on a section of the long-stalled Green Line between Anacostia and Northwest Washington, along with a Red Line spur to Wheaton.

"The disappointment is that we don't have it finished by now," said Carlton R. Sickles, a lawyer and former congressman who has helped plan and oversee the system since the 1950s. "Beyond that, I'm satisfied that it's done what it was supposed to do and improved the quality of life for a lot of people.