The wheeled metal monster known as "Jumbo" is doing battle with bare rock on the tunnel's side, and the noise and vibration are overpowering. Its thin steel arm, tipped by a carbide drill bit, is boring away 200 feet below Georgia Avenue just outside the Beltway, making holes for bolts that will brace the walls of a future Metro station at Forest Glen.

Later, Jumbo's operator turns the bit to the tunnel's face and perforates it with about 135 holes. Explosives are placed inside, then detonated in precise sequence to concentrate the blast where the engineers want it. When the dust settles and workmen step forward, they find a pile of rubble and 10 feet of new tunnel.

Wearing boots and ragged denims, miners slog through ankle-deep mud and breathe air moist with subterranean water that drips from the ceilings. Work proceeds under the pale wash of floodlights, amid the roar of diesel engines and smell of exhaust fumes. "We're not affected by weather here," said a supervisor. "Whether it's raining or snowing or hot, we keep going."

Under a $77 million contract, a work force of 250 is extending the Red Line underground north from Silver Spring toward Glenmont, the line's final stop as mapped out in Metro's full 101-mile subway system. It is one of Metro's deepest and most costly excavations.

But among Metro officials, fear is growing that future construction projects like this one -- financed almost entirely by federal grants -- are in for hard times. First, the Reagan administration refused to commit itself beyond a 75-mile system. Now it is seeking another 12 percent reduction in transit spending. There is serious talk that the full subway system may never be completed.

For now, the money is still flowing. The transit authority remains the Washington area's most prolific builder, employing 4,000 workers and spending about $200 million a year on construction.

Design philosophy has changed markedly, however, since Metro was conceived in the 1960s. Then no cost was considered too great to build a subway worthy of the city's standing as the Nation's Capital. Today, builders are abandoning the frills of a pre-inflationary economy and trying to break ground on unstarted segments as soon as possible.

The Forest Glen and Wheaton stations are cases in point. After an intensive cost analysis in the late 1970s, both were redesigned on the "Stockholm" model. The idea is that two small, single-track stations parallel to each other (with a connecting walkway) are less costly than a single, two-tracked station with its wider base and higher ceiling.

The Red Line terminal station at Glenmont, meanwhile, was reworked to be square and with a lower ceiling, not as pretty, perhaps, as the curves of Metro Center, but, since less digging is required, much cheaper. Metro estimates that the design changes in the three stations will save $40 million to $50 million.

Escalators are another example. Original plans boasted that subway riders would never have to walk up or down a single step. That was abandoned (an escalator for a 20-foot rise now costs about $200,000), so that new stations like Shady Grove on the Red Line will include escalators for trips up, but stairs for trips down.

Metro Center was built with a space between the upper and lower platforms to accommodate extra escalators if needed. Rush-hour crowding on the existing escalators indicates that the extra conduit is needed -- but commuters are to get stairs, not escalators, with work to begin early next year. Estimated saving is $700,000.

Metro Center's vaults are made up of concrete sections that were cast in place. The vault panels for Van Ness and other new stations were precast, trundled below ground and erected like building blocks. They are cheaper (a total $10 million in savings), simpler, and Metro claims, sturdier. To the eye, the effect is more utilitarian, less monumental than the old designs.

A host of smaller revisions, invisible to the rider, were made with little fanfare in the late 1970s. For instance, all tunnel walls originally were to be sprayed with a substance that would dampen noise. That was abandoned to save $7.8 million, with only the four miles of tunnel connecting Dupont Circle and Union Station getting the treatment.

So far, Metro planners have not had to make the ultimate cost cut, the elimination of whole segments of track. But if federal funds do run out, outer segments of the Yellow/Green line to Greenbelt and the Green Line segment into southern Prince George's County appear likely candidates to go.

Reasoning that the federal government would be less likely to abandon unstarted projects than half-completed ones, Metro's eight jurisdictions have agreed on a four-year construction plan that diverts millions from the Glenmont line to get the Prince George's County projects under way.

It also gives substantial funding to start the so-called inner-city Green Line and the Yellow Line spur to Franconia-Springfield in Fairfax County.

Mile-by-mile, Prince George's planned track is much more cost effective than Glenmont's, because it is to run on the surface. That costs $8 million to $10 million per mile (excluding stations), compared to $24 million to $31 million per mile needed for rock tunneling. "The biggest potential for cost saving is your deep tunneling," said John Egbert, Metro's construction chief.

Currently 37 miles of Metro track are in service. Plans call for 70 miles to be operational by 1986, though inflation and the uncertainties of federal funding make such schedules tentative at best. Work now is in progress on five segments:

Dupont Circle-Van Ness, Red Line. Work crews are correcting electrical defects, plugging water leaks and performing final tests. Scheduled to open for commuters on Dec. 6 this year.

Van Ness to Shady Grove, Red Line. The tunnels are finished, with an unbroken passageway extending from downtown to where the line surfaces at the Beltway. Contractors are adjusting elevator designs for Tenley Circle and waterproofing tunnels near the Navy Hospital in Bethesda. Some track has been laid. Scheduled opening date is late 1983.

Silver Spring to Glenmont, Red Line. Construction work has begun only on the segment's southernmost portion, between Silver Spring and Forest Glen. The line tentatively is scheduled to open through to Wheaton in the late 1980s.

L'Enfant Plaza to Pentagon, Yellow Line: Track is being laid but fell two weeks behind schedule due to problems in removing bolts. Wiring continues for the computerized Automatic Train Control system. Scheduled to open late in 1982.

National Airport to Huntington, Blue/Yellow Line. Technicians are ready to "energize" the hot rail of one section of the line and test automatic control systems with real trains. Scheduled to begin operating in late 1982, if enough new rail cars have been delivered by then.

Ballston-Vienna, Orange Line: Three of four planned stations are under construction, as well as service structures along the track's surface route, the Interstate 66 corridor. Scheduled to open in 1986.

Most track still to be constructed will run either on the surface or in comparatively shallow tunnels constructed with the "cut and cover" method, in which a trench is dug and then covered over. The Red Line near Forest Glen, where the machine Jumbo now drills its holes, has some of the last deep rock tunnels that Metro will build.

The miners can make about 30 new feet of new tunnel a day. As blasts extend the station cavern, steel braces are placed in the newly exposed ceiling and spray coated with liquid concrete piped down under force from the surface. The rubble, or "muck", is hoisted to the surface for disposal as road base or fill.

Excavation of Forest Glen's two 600-foot station caverns is about 75 percent completed; two miles of tunnel in the contract are about 65 percent done. By August 1983, contractor Peter Kiewit Sons Co. is scheduled to have put in the "structural" parts of the station -- platforms, concrete vaulting and track bed.

With that finished, the future of the Red Line extension to Glenmont will be less certain. Current plans do not assign significant new funding (assuming federal grants are available) to it until fiscal year 1984. If funding remains short and Metro is forced to delay construction or eliminate segments, the cost of its unstarted rock tunnels between Forest Glen and Glenmont could bring it under attack as an extravagance.