In Metro's longest leap into suburbia, the rail system is set to open four stations Saturday on a Red Line extension stretching along Montgomery County's booming, traffic-clogged Rockville Pike corridor to the Shady Grove terminus near Gaithersburg.

With the completion of the seven-mile, $278 million extension, the Red Line will reach farther beyond the Capital Beltway than any other branch of the rail system. For passengers, the extension will offer the highest fares, newest trains, least frequent service and most parking spaces.

The extension's opening, which was delayed for a year because of a shortage of rail cars, has been applauded by Montgomery County officials, who poured millions of dollars into improved roads and bus service to help commuters get to the new White Flint, Twinbrook, Rockville and Shady Grove stations.

"We're there," exulted Cleatus E. Barnett, a veteran Montgomery County member of Metro's board of directors. "The Shady Grove line is going to be one of our best lines in terms of patronage."

Ridership at the new stations is expected to reach 27,000 trips a day within two months after the extension opens, according to Metro's forecasts.

To attract commuters, the transit authority plans to offer free parking for the first three months at the Shady Grove station's sprawling 3,100-space lots.

The extension also is viewed by county officials as a spur to further development in the upcounty area and as evidence of the soundness of Montgomery's no-nonsense strategy in getting the rail line built. While some local governments were mired in controversy over Metro routes, Montgomery pressed steadily ahead.

"Montgomery selected a route and stayed with it and, by and large, pursued that route single-mindedly," said Edmond F. Rovner, a senior county aide. Now, he added, Montgomery has "got it."

The extension is expected to lure thousands of commuters out of their cars on heavily congested highways. It will link Metro with Maryland's commuter rail station at Rockville. It will offer a new entryway to the fashionable White Flint shopping mall, one-half mile south of the White Flint rail station.

One hint of the new stations' popularity emerged shortly after Metro extended the Red Line in August from the District's Van Ness-UDC station on Connecticut Avenue NW to the Grosvenor stop north of the Beltway, the line's current terminus.

Montgomery offered free parking at the unopened White Flint and Shady Grove stations along with special shuttle buses to link the lots with Grosvenor. Soon the number of commuters parking at the future stations rose to 900 riders a day, a surge that prompted the county to run the buses two extra hours daily.

"That's much better than we had expected," said county transit services chief Edward A. Daniel. "We're regularly carrying standees."

Still, some of Metro's forecasts have proven too optimistic. Officials had predicted that more than 40,000 trips would be taken daily at the stations that opened in August, but recent data put ridership at 34,500 trips a day, a shortfall that led planners to scale back their forecasts for Shady Grove.

With the opening of the Shady Grove extension, the rail system will encompass 57 stations and 60.5 miles of tracks, well over half the 101 miles envisioned by local officials.

The Red Line already is the longest line in the rail system, and its western branch will stretch 17.9 miles from Shady Grove to the Metro Center station in downtown Washington, farther than any branch open or any planned. Next longest is the planned 16.2-mile Yellow Line branch connecting the District's Gallery Place station with Fairfax County's proposed Franconia-Springfield terminus.

Largely because of concern over high fares on the long Shady Grove route, the Metro board recently clamped a $2.40 lid on rush-hour trips. Without the cap, the price of riding from Shady Grove to any station beyond the Cleveland Park stop on Connecticut Avenue NW would have approached or exceeded $3, according to the mileage formula currently used for rush-hour fares.

At times other than rush hours, the maximum rail fare is $1.10. The minimum fare for short trips at rush and nonrush hours is 80 cents.

A prolonged controversy surrounded the issue of how often the trains should run on the Shady Grove extension. Montgomery officials pressed for frequent service, while other officials expressed concern about high costs and rail car shortages. In the end, they compromised.

When the extension opens, trains will run every 10 minutes at rush hours between Shady Grove and Grosvenor and every five minutes between Grosvenor and Silver Spring. Rush-hour service will be increased in two steps during the next three months.

On Jan. 6, trains will start running every eight minutes at rush hours between Shady Grove and Grosvenor and every four minutes between Grosvenor and Silver Spring. Starting March 17, trains will operate every six minutes between Shady Grove and Grosvenor and every three minutes between Grosvenor and Silver Spring.

During evenings, midday and weekends, trains are to run every 12 minutes between Shady Grove and Grosvenor.

The Red Line schedule contrasts with trains on the outer sections of the Blue, Orange and Yellow lines, which operate every six minutes at rush hours and every 12 minutes during nonrush-hour periods. At points where the Blue Line overlaps the Orange or Yellow lines, trains run every three minutes at rush hours and every six minutes at other times.

To limit possible crowding, Metro officials suggested that commuters riding from downtown Washington to the new Red Line stations take whichever train shows up first, whether it is bound for Shady Grove or Grosvenor. If it is a Grosvenor train, riders may transfer to a Shady Grove train at any station.

Because of continuing rail car shortages stemming from delays in testing newly assembled cars, Metro officials have warned of possible temporary crowding on some Red Line trains after the extension opens this month and, again, after bus routes are revised to serve Red Line stations in January.

In addition, the rail system plans for the first time to operate some newly delivered rail cars with special electronic devices designed to reduce electric power consumption. This new equipment has posed complex problems, and officials have said they are uncertain whether the cars will be ready for use.

If those cars are not available, officials said, the rail system may have to reduce its reserve fleet of spare cars, a move that might eventually jeopardize the dependability of rail service.

Several other problems have been cited by Metro and Montgomery officials. At White Flint, construction of a second parking lot has been delayed because of negotiations with developers. At Twinbrook, an entrance ramp for elderly and handicapped riders needs to be rebuit because it is too steep, officials said.

At Rockville, a glass-enclosed overpass designed to allow Metro patrons to cross congested Hungerford Drive (Rte. 355) is surrounded by construction work because of extensive renovation at the old Rockville Mall. At Shady Grove, some highway improvements are years away from completion.

Officials also have issued warnings because of incidents in which intruders have climbed over or cut through a fence at Frederick Avenue north of the Rockville station to walk across Metro tracks. Crossing tracks is hazardous because of the 750-volt third rail and automatic track switches, officials said.

While parking will be free for the first three months at the two Shady Grove lots, the largest in the Metro system, the transit authority plans to charge a $1-a-day parking fee at the 930-space White Flint lot, the 500-space Rockville lot and the two Twinbrook lots, which include 1,000 spaces.

Because of concern about a possible influx of commuters, the Rockville city government has restricted parking on streets near the Rockville and Twinbrook stations to residents with special $10 permits from 8 a.m. to 5 p.m. on weekdays. Fines for parking violations will be $25 for a first offense and $50 for subsequent offenses, a city spokesman said.

Major changes in bus service provided by Metro and Montgomery's Ride-On system have been delayed until next month, partly to avoid confusion for commuters and possible crowding on trains. The realignment has been described as the biggest expansion in Ride-On's nine years of operation.

The shifts, scheduled to take effect Jan. 27 and 28, include new and revised bus routes serving Red Line stations. In addition, some Metrobus service is to be curtailed on routes that parallel the rail line to reduce what are viewed as excessive costs.

Starting Saturday, the White Flint Mall plans to operate a 15-passenger shuttle van, charging 25 cents a ride, between the White Flint rail station and the shopping plaza. Vans are scheduled to run every 30 minutes between 9:30 a.m. and 9:25 p.m. Monday through Saturday.

Maryland officials said they plan no immediate changes in service on the state's commuter rail line between Brunswick and Union Station, which includes 10 trains a day stopping at Rockville. Amtrak also offers limited commuter service with a Rockville stop.

"The demand is out there. The commuters very much want the service to go on," said State Railroad Administration spokesman Kathleen Kujawa. "We are here to stay."

Roads have been built and many streets have been widened, extended or repaved to accommodate commuters driving to the new stations, especially in the Shady Grove area.

Nevertheless, officials cautioned that delays in some key projects may lead to congestion.

A proposed highway known as I-370, designed to allow Shady Grove commuters to bypass the congested intersection of I-270 and Shady Grove Road, is not scheduled to be built until 1989.

One section of a new Mid-County Highway linking Shady Grove Road with Laytonsville Road (Rte. 124) is complete, but an extension to Montgomery Village Avenue is not expected for several years.

Several major development projects are under way near the new Metro stations, including an office and retail structure known as White Flint North.

The project, being built by a group headed by the Lerner Corp. south of the White Flint station, is expected to be completed in about a year.

Metro officials have engaged in protracted negotiations with another development group, headed by Blake Construction Co. Inc., and Richmarr Construction Corp. over a proposed $225 million office, hotel, retail and condominium complex near the White Flint station.

NEXT: The ambiance of White Flint