At 10 minutes till 8 one recent Friday morning technicians at Metro's control room put in a call for the ready reserve: a "gap" train was needed in a hurry.

A rush-hour train had broken down on the Red Line. Sending in a standby train was the fastest way to plug the gap in subway service.

The maneuver provided a quick fix: Just nine minutes later, the gap train pulled out of Silver Spring, carrying commuters downtown. The Red Line was back on schedule. Only a few passengers had been inconvenienced. Thousands of other riders were spared annoying delays and overcrowding.

Metro's gap train gambit represents one tactic in a far-reaching campaign that has led to a marked improvement in transit service in the past three years. In 1982, some officials feared the subway system had begun a "disastrous" decline. Today Metro is racking up records for higher reliability.

"We are getting a little bit smarter," said Fady P. Bassily, Metro's assistant general manager for rail service.

According to Metro officials, the turnabout has resulted from a series of shifts, including day-to-day use of the gap trains. Malfunctioning equipment has been overhauled. More subway cars are held as spares. Maintenance has been revamped. Responses to breakdowns and disruptions have been revised.

One key index, the average distance a subway car travels without a serious delay, has nearly tripled in three years, reversing an earlier slump. Subway cars now average more than 25,000 miles without a major incident, compared to about 9,300 miles in fiscal 1982. The monthly index hit an all-time high of 34,846 in March.

The current 25,403-mile average means that on a typical day fewer than four delays occur anywhere on the system, officials said. Included in the index are delays lasting five minutes or more caused by mechanical malfunctions, power outages, severe weather and incidents such as suicide attempts.

Metro's reliability push has not been aimed solely at the subway system. Steps also have been taken to make bus service more dependable, such as renovation of older buses and changes in maintenance. Breakdowns have been reduced, notably on routes serving low-income District neighborhoods.

Other rail and bus equipment, including the subway system's trouble-plagued Farecard apparatus, has been slated for upgrading.

"Reliability is probably the best it's ever been," said Metro General Manager Carmen E. Turner. "I think we've just overcome all sorts of obstacles."

Richard S. Page, Turner's predecessor, launched the dependability drive, warning in a September 1982 memo that Metro had been hurt by "a continued decline in service reliability, particularly on Metrobus." Turner, who took over in mid-1983, gave the issue renewed emphasis.

Recently, however, officials have expressed increasing concern because of moves by the Reagan administration to reduce federal outlays for transit systems. Some steps taken by Metro, such as buying more reliable equipment, have been financed with federal funds, and officials said cuts may hamper similar efforts in the future.

Since the deployment of gap trains as part of an experimental revamping of subway operations in January 1983, they repeatedly have been cited as a "critical element" in day-to-day service. Although gap trains had long been used by other transit systems, Metro previously employed them only sporadically.

The Red Line gap train incident in March was cited by Metro officials as typical. At 7:46 a.m., train No. 104 had been crippled by a loss of propulsion, a routine malfunction, as it reached Union Station. Within minutes, No. 104 was ordered to a repair yard, leaving a gap in Red Line service.

If the gap remained, the line's tightly charted schedule would have been skewed throughout rush hour. Many passengers would have faced at least short delays. The train behind No. 104 probably would have become overcrowded. The crowding likely would have slowed service further.

Instead, the control room radioed Silver Spring, where one of Metro's six strategically targeted gap trains was on rush-hour standby. The gap train quickly swung into operation. It picked up passengers at Silver Spring and then headed downtown at 7:59 a.m., following No. 104's schedule to the minute.

The only passengers inconvenienced by the breakdown were those forced to get off the disabled train at Union Station and those waiting for an outbound train toward Silver Spring in the next few minutes. Since most riders travel on inbound trains at that hour, the impact was relatively slight.Corrective Action

Other shifts cited by Bassily and other Metro officials as helping to make service more reliable include: Slower acceleration by subway trains. To lessen wear and tear on motors and other equipment, officials last year cut by half the rate at which trains accelerate. In addition to reducing breakdowns, the cut also was aimed at decreasing electric power consumption. To offset the reduced acceleration rate, Metro slightly increased the time trains travel at high speeds. This move ensured that subway trips would not take any longer than before, officials said. More spare trains. In addition to the gap trains, Metro has increased its spare fleet to about 20 percent of the active rush-hour fleet. The aim is to have enough spares to allow time for preventive maintenance. The spares also are used to replace gap trains called into service. In the past, the "spares factor" often had dropped below 20 percent. When there are too few spares, said Bassily, "you pay for it." Not unloading passengers for minor problems. To reduce disruptions, Metro last year revised policies to allow a train with a minor malfunction to stay in service until reaching a terminus, where it could immediately be replaced by a gap train. Previously, passengers were ordered off trains with minor problems, such as loss of power in one of six cars. Officials said the policy applies only if there is no safety risk. More Monday morning quarterbacking. An "incident analysis" system was instituted to try to improve Metro's response to disruptions. Bassily said this system has led to "tremendous improvement" in the way split-second decisions are made. The issues range from dealing with a tunnel flooded by a broken water main to determining whether a train's motor is getting overloaded and figuring when to avoid time-consuming "single tracking" around a stalled train. Cracking down on off-schedule trains. Since mid-1984, Metro has tightened curbs on delays caused by train operators who wait too long at stations. Supervisors have been assigned to monitor this "dwell time" closely and to alert operators who let their schedules slip. Overhauls of faulty equipment. Some 300 traction motors found to cause frequent breakdowns have been rebuilt. Resistor grids that repeatedly overheated in propulsion systems were modified. Rapidly wearing brake components were replaced on more than 40 cars. Faulty train control relays were replaced.

Major new efforts now are under way, including a federally financed $35 million renovation of train equipment ranging from trouble-prone motors to malfunctioning destination signs. To reduce cold weather problems, officials are considering changing lubricants that get too thick in winter and modifying thresholds that cause doors to stick.

"We are finding the problems before it hits you in service," said Bassily. "We are maturing."