The bus system, long the mainstay of the Washington area's mass transit network, has been overshadowed in recent years by Metro's electronically dazzling, multibillion-dollar subway. But now, officials say, bus service has begun to get renewed attention and some much-needed improvements.

"We're starting to take a real hard look at the bus 'company,' " said Metrobus chief Shirley A. DeLibero.

The emphasis, DeLibero added, is welcome. "This has always been a railroad."

The sharpened focus has helped reduce bus breakdowns and delays, according to Metro officials. Buses are cleaner, they said. Steps also have been taken to bolster service in low-income D.C. areas, including Southeast Washington, where many residents depend heavily on public transportation.

The improvements are largely the result of extensive efforts to increase maintenance and renovate older, trouble-prone buses, officials said.

With more than half the planned 103-mile subway system now in operation, Metro officials have shifted their attention, to some extent, from rail construction to transit service.

Both bus and subway service appear to have benefited from the heightened concern.

Metrobus service has broad impact partly because bus ridership continues to outstrip subway patronage.

Bus ridership averages about 430,000 trips a day, compared with 370,000 on rail. Buses serve major corridors and widely scattered residential neighborhoods. And many commuters ride buses to rail stations. Maintenance Blamed

Concern reached a peak three years ago, after what was viewed as a steady deterioration in bus service.

An internal study in mid-1982 linked the decline largely to inadequate preventive maintenance, a shortage of mechanics, lack of critically needed spare parts and insufficient training.

Since then the authority has sought to reverse the slide and, according to one key indicator, bus service has shown a more than 60 percent improvement. Average mileage between bus breakdowns has climbed from 959 miles in fiscal 1982 to 1,598 this year -- a level approaching that recorded in the late 1970s.

The upturn has been more pronounced on Southeast Washington routes, the statistics show. Even though service remains less dependable there than in suburban areas, mileage between breakdowns in Southeast has risen by 77 percent in two years, from 613 miles in fiscal 1983 to 1,085 now.

Officials have attributed the higher rate of malfunctions in Southeast and other urban areas to more extended hours and more severe driving conditions than on suburban routes.

Stepped-up maintenance has played a major role in Southeast and throughout the system, officials said. Thirty mechanics were shifted to two D.C. garages, including one in Southeast, from suburban garages nearly two years ago. This year, Metro added 25 mechanics to its garage staff, the first such increase in recent years.

A series of new and beefed-up inspections at garages has been instituted, including weekly checks of brakes and other safety-related components, along with more comprehensive examinations every two months or 5,000 miles.

Monthly audits of garage work were started. Lab tests of oil and other fluids were begun in an attempt to spot engine and transmission problems before breakdowns occur. Training was expanded, and a second maintenance shift was added at the large Bladensburg garage in Northeast Washington.

Recent overhauls of several hundred older buses also have helped reduce breakdowns, officials said. Last year, a $21.7 million renovation of 260 General Motors Corp. buses was completed for Metro by a Chicago company.

Metro recently has replaced critical parts on more than 300 Flxible Co. buses. Crackdown on Employes

Metro officials also have attributed the improvements to several other measures, including curbs on absenteeism by drivers and mechanics, a crackdown on alcohol and drug abuse, decentralization of bus management and posting more supervisors along routes to help keep buses on schedule.

Despite the moves, key problems remain, including some that have persisted since Metro took over the area's private bus companies in 1973.

The bus fleet is aging; some garages are dilapidated; routes are circuitous and frequently revised, and service on some lines is erratic.

One recent move went awry. Metro bought 76 buses from Neoplan U.S.A. Corp., a West German-affiliated company, in an attempt to replace some older vehicles.

In February, however, the Neoplan buses were taken out of service because of cracks in their undercarriages. Officials recently said they are close to an agreement on how to repair the buses. Banking on More Buses

To strengthen service in the next few years, officials are banking heavily on buying more buses. But prospects are uncertain, partly because of limited funds.

"We've got the oldest buses in the entire country," said DeLibero, citing statistics indicating that the average Metrobus is more than nine years old, about 1 1/2 years older than the U.S. average. "My goal is to have the average of the fleet be around six years."

Outmoded maintenance garages have been another longstanding concern. Metro has failed in repeated attempts since the 1970s to find a site to build a replacement for its cramped, ill-equipped Southeastern garage at M and Half streets SE. Plans for other garages also have been delayed.

Meanwhile, the authority's most modern garage, the $17.6 million Montgomery County facility, remains severely underused.

Officials said the facility was planned before the county's Ride-On bus system supplanted Metrobus service on many Montgomery routes.

Metro's frequently changing routes also remain an issue. Consultants' recommendations for a sweeping realignment of routes were shelved in the 1970s.

Since then, routes have been revised almost piecemeal in attempts to cut costs, tie bus service to Metro's expanding subway system and mesh with new county- and city-run bus systems.

Despite the recent steps, DeLibero said, "it's going to take a while to get the riders back that we lost."