Metrorail's reputation for speed and reliability is in trouble.

Since last December, trains have been breaking down with unusual frequency, stranding passengers on crowded station platforms and slowing other trains on the line.

With ridership lagging 10 percent behind expectations, many top managers at Metro have reached the bitter conclusion that their six-year-old subway system, built as a showpiece for the country, is driving some of its riders away.

Some major factors behind the service problems:

* Metro traditionally has worried more about building the rail system than about running it efficiently.

* Racing-car speeds and quick braking, designed into Metrorail in hopes of lowering trip time and drawing riders, place enormous strain on the trains.

* The cars' "state of the art" technology brings with it a host of special maintenance needs. Even conventional parts, brakes in particular, have had bizarre ailments that baffled mechanics for months.

* Train operators have taken a "play it safe" attitude since the Jan. 13 derailment that killed three people. Trains go out of service at the slightest sign of trouble.

* The fleet was stretched thin when Metro decided to open the Red Line's Van Ness extension last December without additional cars.

* The maintenance force has been constantly under strength. Repair work is done in facilities that everyone concedes are too small.

Staff planners are preparing to present to the Metro board a $30 million, three-year program to improve critical parts in the cars. Board members now are particularly receptive, having formally acknowledged recently that Metro has given short shift to service reliability from the start.

"We're going to be in the woods for a while," said Metro operations chief Theodore Weigle.

Since service began in 1976, Metrorail has met and mastered repeated maintenance problems. But performance began going downhill in a big way last December. This spring, things recovered somewhat, but remain well below last year's averages and the levels that Metro wants.

In fiscal 1981, Metrorail completed an average of 98.5 percent of scheduled trips. In January it dropped to 96 percent, then began edging back up, before falling off again in June. The percentage of trips delayed five minutes or more averaged 0.83 percent in fiscal 1981. It hit 1.3 percent in January, improved in the spring, then rose again, touching 1.1 percent in June.

Metrorail officials say some of these shortcomings grew from a conscious decision to treat every mishap as an emergency. Before the January accident, smoking brakes often were treated as harmless, they say. Now the train is put out of service immediately. "We treat smoke as if it were fire," said operations official Mark Miller.

Incidents in which passengers had to leave problem trains rose earlier this year. Then Metro began holding back cars that were in borderline condition, and the number of incidents dropped, according to Weigle.

That has exacted a price, however: Metro has met its rush-hour requirement of 256 cars less and less frequently, meaning cars that do run tend to have more and more people in them.

Also contributing to the poor rush-hour showing was Metro's gamble last December to expand service to three new Red Line stations in Upper Northwest without new cars. That made the reserve fleet smaller, meaning Metro is less able to find substitutes for inoperative cars.

Deterioration in service was a major factor in 1982's disturbing ridership figures, many officials believe. Metrorail is now providing about 300,000 rides per day, 25,000 fewer than were expected. April and July's showings were actually below figures for those two months in 1981, despite the opening of the three new stations.

Good rail service needs extremely tight standards of maintenance. A single broken bus is towed to its garage with little effect on others on the street. One broken railcar, on the other hand, can render an entire train useless, and in turn back up an entire line.

If brakes on a railcar headed for National Airport start smoking at Foggy Bottom during the evening rush, for instance, the whole train's load -- there may be 1,000 people or more aboard -- must get off. Trains behind it all the way to Addison Road and New Carrollton may be slowed as they await a turn to "wrong-rail" through Foggy Bottom on the one open track. That in turn disrupts traffic the other way.

Soon crowds of impatient commuters are collecting on platforms up and down the Blue/Orange Line, as familiar announcements that trains are "experiencing delays" are heard. Meanwhile, control room technicians and mechanics are trying to get the dead train off the track and unplug the jam. Some days can be particularly bad. They had to deal, for example, with 13 dead trains on July 20, and 14 on July 22.

At Metro's Brentwood maintenance shop, which trains pass between Union Station and Rhode Island Avenue, mechanics and supervisors say they do their best with what they have. Cars are aging and temperamental and spare parts can be in short supply. "Everything's coming up on four- or five-year overhaul," says mechanic Ed Hiller.

The Metrorail car was billed as one of the world's most modern mass transit vehicles when it entered service six years ago. Electronic equipment handles many functions assigned to humans in older-model cars. When a train runs on full automatic, the operator has little more to do than announce stations, open doors and watch for signs the computers are bad.

But keeping the cars railworthy is a constant battle. For instance, in two cars (they operate as permanently coupled "married pairs" because each one contains components its mate does not have), there are 31 separate electric motors driving such things as wheels, air conditioners, compressors and cab defrosters.

Fourteen percent of 710 breakdowns logged in April, for example, were blamed on problems in the cars' computer-based Automatic Train Control and communications gear. Eighteen percent were traced to failures involving "logic packages," complex electronic boxes that regulate how much power reaches the motors during acceleration and stopping.

Metrorail's electronics shop at Brentwood is now handling a steady stream of electronic circuitry getting major overhauls after long use. "It's just like your television. Five years and you don't think anything of taking it to the shop," says shop supervisor Charles Hallock.

Heating, ventilation and air conditioning equipment accounted for another 11 percent of the failures.

Though advanced equipment needs close care, many failures involve systems basic to any railcar, in particular brakes and the electric traction motors that drive the wheels. Of the breakdowns in April, 22 percent were related to brakes and 6 percent to traction motors.

The search for better brakes has consumed millions of dollars and countless hours of mechanics' time.

Passengers remember well the painful screams brakes emitted in 1980. That noise was caused by discs and wheels "singing" when brake pads were applied to generate friction for stops. After months of work, the noise was stopped by installing different pads.

In the meantime, Metro had decided to replace the discs with another model, which seemed to last longer. But after the first of these German-made discs were installed last fall, the trains were hit by a rash of mysterious "fires."

Passengers would see smoke beneath a train and notify the operator, who would unload the train (creating yet another traffic tie-up) and call a repair team. In many cases, though, mechanics could find nothing wrong when they got there.

Metro finally fingered a suspect: the preservative put on the discs for their Atlantic crossing. That was removed but the smoke continued. A new theory evolved: blue paint on the discs was blistering and smoking at relatively low temperatures. By now it was spring, and countless riders had gotten first-hand experience with the brakes' problems.

How to remove the paint? Mechanics at the Brentwood maintenance yard tried sandblasting and blowtorching and finally settled on baking the discs all night in a large electric oven. They're still doing that and the problem of smoking dics are fading, Metro officials say.

In traction motors, the problem is failure of basic parts. The motors -- they look like boulder-sized versions of the ones that power table fans -- were meant to last the promised 30-year life of a car but in fact start to fail after they run up 60,000 miles (one year's service at current rates), Metro says.

Cracks have developed in metal commutator plates that rotate at high speed inside the motors. The banding that holds them in place has proven too weak. The motors get "out of round," resulting in rubbing and friction inside and eventual breakdown.

Metro has pinpointed design faults and poor quality control at Westinghouse, which produced the motors. James F. Mullervy, regional transportation sales manager for Westinghouse, does not directly dispute that contention and says the company has offered to pay for new plates and banding for Metro.

Mullervy and others say designers may have underestimated how punishing Metrorail's high-speed performance would be. "You either operate full power or full brakes," says Erich Vogel, Metrorail's maintenance chief. In some places, between Silver Spring and Rhode Island Avenue, for instance, trains hit 72 mph, more than 15 mph faster than top speeds at older systems such as New York's.

The breakdowns have led some Metro officials to question whether so much speed and sophisticated gear is a good thing. Weigle points as an example to Montreal and Toronto, where fewer-frills systems provide what many in the transit industry rank as the best rail service in North America.

For that reason, Metro is giving thought to slowing down its trains a bit. Such a decision would not come easily, as it would mean changes in scheduling, Automatic Train Control programming and an operating philosophy sworn by since Metrorail was designed in the 1960s.

Unlike Metrobus garages, where maintenance work is routinely put off or never done at all, Brentwood has kept standards for preventive work fairly high, Metro officials say. Still, rail operations chief Joe Sheard says he would like to see less "firefighting" and more attention to routine work at the yard.

The future holds promise but potential for new problems too. The $30 million maintenance plan about to go to board members could reduce current failures. And the opening next month of a large yard in Alexandria for light maintenance should relieve pressure on Brentwood, which was intended as a heavy maintenance center only.

Staffing on the maintenance floors, meanwhile, is finally approaching 100 percent of authorized levels. The shops have been well under strength for years, due to difficulties in finding qualified people (many who were hired are former military aircraft mechanics who came with no experience with railcar maintenance) and other problems, Metro officials say.

Officials hope that delivery next year of the first of 300 new railcars being built by the Italian company Breda Costruzioni Ferroviarie will also improve overall fleet performance. Metro has ordered countless changes in the cars' design to try to correct the current cars' deficiencies.

However, the Breda cars, designed exclusively for Metro, will arrive essentially unproven and could well develop a list of troublesome quirks all their own.