A small trash fire in the subway tunnels under the Potomac River yesterday morning brought trains to a halt, delayed thousands of commuters and raised anew questions about the adequacy of Metro's preparations to fight fires.

No one was injured.

Some commuters waited for the hour-long problem to clear up, others hiked across Memorial Bridge or, alerted by news broadcasts, jumped in their cars to create another memorable clear-day traffic jam on the Potomac bridges.

Among the problems that occurred yesterday were:

Metro dispatched a train that was carrying passengers into the smoke-filled tunnel and stopped it at the site of the fire so a supervisor could investigate.

Both the District of Columbia and the Arlington fire departments were called to the wrong places to deal with the fire, perhaps because its location was not immediately determined.

Arlington firemen first learned the precise location and nature of the fire from a report on radio station WMAL, not from Metro.

When firemen reached the blaze and attempted to "charge" their hoses from a water main, the main sprang two leaks.

Radio communication from the tunnel was difficult and there was no direct communication between the Arlington and D.C. departments. The D.C. department withdrew when it heard from Metro that Arlington was fighting the fire.

A full internal investigation was launched by Metro yesterday. "We really don't know a lot of answers yet, and those are some of the things we're looking at," said Nicholas Roll, Metro's assistant general manager for transit services.

The fire consumed only trash and oily rags and did no damage to the tunnel or equipment in it, according to Metro officials.

The blase was located in the so-called midriver pumping station, which is 2,739 feet -- about half a mile -- from the end of the Rosslyn platform. Two large pumps there -- the lowest point in the tunnel between Arlington and D.C. -- discharge water that seeps into the system.

The first report of trouble came at 6:34 a.m. in a radio report from a train operator to Metro's control room. The operator said he had seen smoke in the tunnel. Within five minutes, station attendants at both Rosslyn, on the Virginia side, and Foggy Bottom, on the District side, reported smoke in their stations. Both the Arlington and the D.C. fire departments were called by Metro to the Foggy Bottom and Rosslyn stations.

Normal procedure, according to both Metro and fire departments, calls for the fire departments to be summoned to vent shaft openings on either side of the river that are much closer to the pumping station than either the Rosslyn or Foggy Bottom stations.

"The biggest problem I see is the vagueness of our instructions," said R. S. Carpenter, deputy fire chief for Arlington. "All we were told about was 'smoke in the Rosslyn station.'"

Metro train supervisor Charles Hunter was dispatched from the Foggy Bottom station on the next train through to Virginia to see if he could find the trouble.

According to Roll, Hunter got off the train at the pumping station, found the smoke too heavy to remain, boarded the train again and continued to Rosslyn.

An undetermined number of passengers were on that train with Hunter. It is likely that the train carried a light load because it was heading away from downtown.When asked why the train was not unloaded at Foggy Bottom before Hunter went down to investigate, Roll said, "That's one of the things we're checking."

Hunter's train was the last through the tunnel. Four other trains -- two at Rosslyn and two at Foggy Bottom -- were backed out of those stations so the third-rail electrical power could be turned off.

While firemen walked into the tunnel, there was chaos elsewhere. Washington-bound commuters found themselves standing on platforms at Clarendon on the Orange Line and Arlington Cemetery on the Blue Line with no place to go just as the rush hour was beginning in earnest.

The system continued to operate normally on the entire Red Line, and on the Blue-Orange lines from New Carrollton to McPherson Square.

Metro provided some bus service for Virginia-D.C. commuters. About a dozen buses carried a total of 1,000 people in several trips, officials said. Some other buses at the Pentagon, National Airport and Ballston were instructed to continue into Washington.

At Arlington Cemetery, many people walked up to Memorial Drive, then across Memorial Bridge. Others hitch-hiked. Hundreds waited.

After the problem was over, at about 8 a.m., Metro ran an empty train into Arlington Cemetery to clear the platform.

By 8 a.m., hundreds of commuters had heard of the trouble on their radios and had elected to drive. Just as happened March 11, when Metro's master control panel suffered a power failure and stopped the entire subway system, traffic was a mess on the Potomac bridges. Shirley Highway and the George Washington Parkway were particularly hard hit with rush-hour traffic jams.

Although yesterday's fire was much less serious than the earlier shutdown, the problems in communication and coordination among Metro and fire departments are similar to those encountered in January 1979, on San Francisco's Bay Area Rapid Transit (BART) subway.

An Oakland fireman was killed as he stumbled through a toxic-smoke-filled tunnel while attempting to find an emergency exit, and at least two trainloads of BART passengers were exposed to the blaze. The San Francisco Fire Department extinguished the fire after a grueling and confused seven-hour battle.

As a result of BART's experience, Washington area fire chiefs renewed a longtime call for improved communication equipment in the Metro tunnels, better markings in the tunnels and better maintenance of fire protection equipment.

Metro's then-new general manager, Richard S. Page, promised action on several fronts. A radio system that will permit interdepartmental communication underground is being designed. A phone line for exclusive use of fire departments in the tunnels is being installed and should be operational in a few months. Improved markings have been painted on the tunnel walls, although they were of little value in yesterday's fire.

Arlington is the only fire department with a capability of radio communication in the Potomac tunnels at the moment, and Carpenter said yesterday that system did not work as well as it should have.

That leaves the question of Metro's operating procedure, which permitted the dispatch of a passenger-carrying train into a smoke-filled tunnel.