In a huge cavern blastered from soled rock, at the foot of a deep shaft that resembles a playground slide where a giant's children might cavort, the first Metro trainload of passengers arrived in Arlington yesterday.

The cavern is Rosslyn station, 97 feet below ground, a spectacular example of subway construction, linked to the surface by the longest escalators in the world outside the Soviet Union - 194 feet from end to end which, if placed in an office gap building, would reach without a gap from the lobby to the 10th floor.

The escalators occupy a huge oval shaft, burrowed at a 30-degree angle downward from the street surface.

For about 50 people who took a preview ride from the Metro Center station at 12th and G Streets NW past Rosslyn to Arlington Cemetery station yesterday, Rosslyn was the high - or, in a more literal sense, the low - point of a trip that made local transit history.

Although test trains have been running on the tracks 101 feet beneath the Potomac River surface since Dec. 17, yesterday's journey was the first on which passengers, other than Metro technicians, were aboard. The riders were Metro board members, media representatives and transit authority officials.

The 3 1/2-mile segment they rode is part of Metro's second line, 12 miles long, that is scheduled to go into public service July 1.

When that happens, the subway system will total 17.6 miles of an ultimately planned 100 miles, and will have two routes intersecting at Metro Center.

The original line, now in its 10th month of public operation, will continue to run from Rhode Island Avenue station through downtown, but its terminal will be moved from Farragut North to Dupont Circle. The second line will run from Stadium-Armory to National Airport.

Up to now, test trains have run on the second line for the entire distance from L'Enfant Plaza in Southwest Washington to the portal of a tunnel near the Pentagon in Arlington County. The first such trip under the river was made Dec. 17. Yesterday's trip covered the central portion of the previously tested alignment.

When the public gets aboard the new line, a journey from Metro Center to the Rosslyn station in Arlington will take 7 minutes, compared with up to 30 minutes it now takes a rush-hour bus to go through Georgetown and over Key Bridge. Trains will exceed 70 miles an hour for the underwater part of the trip.

But yesterday's journey was slow, never exceeding 15 miles an hour, a caution that proved its worth when the train clipped the corner of a temporary safety-barrier gate near Rosslyn, putting a minor crease in the car's aluminum siding.

The train also made a stop yesterday that never will appear on a published timetable. It was under the river, midway between Key Bridge and the northern tip of Theodore Roosevelt Bridge, where the subway crosses the D.C.-Virginia boundary.

Since there were politicians and television cameras aboard, it was the logical place for impromptu speeches.

Rep. Herbert E. Harris II (D-Va.), a former member of the Metro board who would have been its chairman in 1975 if he had not been elected to Congress, recalled a walking tour Oct. 7, 1973, that stopped at the same place in the then-trackless tunnel.

Calling yesterday's trip symbolic, Harris said he was "probably as excited as I've ever been before" at Metro's potential for providing convenient commuting and environmental benefits.

Harris promised full support for completion of the system, thus providing the only oblique reference during the trip to the fact that Metro is in financial and political trouble, and may never be completed.

Moments later, when the train arrived at Rosslyn, somebody pointed to the junction where a future line will branch off westward beneath Arlington and - perhaps - will occupy the median of a new Interstate Rte. 66 extending all the way to Vienna.

U.S. Secretary of Transportation William T. Coleman Jr. cleared the way for I-66 and the Metro extension on Wednesday by authorizing construction of the controversial highway and insisting upon state financial support for the Metro line.

For Virginia, the arrival of the first passenger-carrying Metro train has a little-realized historic significance. It restored electrified transit service to the state that is regarded as its birthplace.

For it was 90 years ago that inventor Frank J. Sprague installed, in Richmond, the world's first successful electric street railway system, using basic technology that - much refined - still propels today's sleek Metro cars. The Richmond trolleys disappeared in the 1940s.

And, by coincidence, the last trolley ever to run in Virginia ran on a surface route from downtown Washington that passed above the location of the new Metro Center subway station and terminated at loop just two blocks from metro's Rosslyn station. It made its last trip Aug. 26, 1956.

By anybody's standards, the new Rosslyn station is an impressive, even a spectacular, example of subway construction.

Blasted from solid rock it is a split-level station with Virginia-bound trains occupying the lower level and Washington-bound trains using the upper.

This design is necessary, according to Roy T. Dodge, Metro's chief of design and construction, because Rosslyn will be a junction of two subway routes. Metro engineers decided from the outset that there would be no point on the system where passenger-carrying trains moving in one direction would cross the tracks of trains moving in the other.

The result is the railway equivalent of a freeway interchange, moved deep underground.

A visitor can see the depth, looking either up or down the escalator shaft that burrows downward from North Moore Street near Wilson Boulevard.

These escalators, Dodge said, are the longest in the world outside the Soviet Union, where marshy conditions forced Leningrad to build one 350 feet long.

Washington's Metro will have two stations deeper than Rosslyn, but neither of them, Zoological Park or Glenmont, will be equipped with such long escalators.

Future passengers who ride the line traversed yesterday will find that, in general, the new route is a functional and visual carbon copy of the first one.

At the Metro Center station, the line occupies the bed of 12th Street one level below the existing G Street subway. Trains bound for Virginia go north, swing westward under I Street NW and pass through three stations - McPherson square at 15 Street; Farragut West at 17th Street, and Foggy Bottom at 23d Street - before plunging beneath the Potomac for the 1.4-mile underwater run to Rosslyn. Trains exit from the tunnel before arriving at Arlington Cemetery.

At the deepest point under the Potomac, the trains are beneath 18 feet of water, 10 feet of mud and 73 feet of rock.