A Metro maintenance employe was suspended yesterday because he was found to have been smoking in the subway tunnel under the Potomac River shortly before the outbreak of the trash fire there closed the subway and ruined the rush hour Wednesday morning.

Metro General Manager Richard S. Page confirmed the action, but said "we don't know that is what caused the fire." The employe was not identified. "

Page, who was out of town Wednesday, returned yesterday to find a full-scale internal investigation under way, a call for congressional hearings and the high probability of National Transportion Safety Board hearings into the question of transit fires and fire protection techniques generally.

The cause of the fire still was undetermined yesterday.

Metro and Arlinton fire department officials said yesterday that a maintenance crew had visited the pumping station where the fire occured shortly before its outbreak. Two pumps, which discharge water that seeps into the tunnels, are serviced every day.The two men who service the pumps were picked up at the pumping station by a work train about 6:30 a.m., 13 minutes before the first report of smoke reached Metro headquarters.

Anthony J. Stefanac, the general superintendent of the rail system, yesterday defended the actions of his supervisors after smoke was reported. He said that, after the reports were received, one supervisor made a round-trip by train from Foggy Bottom to Rosslyn and back, but saw no heavy smoke or fire.

Then, Stefanac said, the supervisor decided to stop the next train at the pumping station to check there. It was there that the fire was spotted.

Passengers were carried on these trains.

"We're damned if we do and damned if we don't," Stefanac said. Smoke, he said, is reported in the tunnels from time to time and "we can't shut down the subway every time we get a report."

He said that the ventilating system on the trains was closed to prevent outside air from coming inside after the first report of smoke. Further, he said "we have received no complaints from passengers."

Rep. Herbert Harris II (D-Va.) called yesterday for oversight hearings by the House District of Columbia committee into Metro's fire preparations. The fire, he said in a letter to committee chairman Rep. Ronald Dellums (D-Calif.), "raises serious questions about Metro's ability to react quickly and appropriately in time of emergency."

Yesterday, two members of the National Transportation Safety Board said that the entire question of transit safety was appropriate for a major board study.

Member Patricia Goldman said. "It has occurred to me as I've ridden [Metro] that I wonder how in hell I would get out. There are no evacuation instructions or public education program."

Board Chairman James King said the board is considering "some kind of symposium on having a fire safety program that works. We've seen fire problems in Philadelphia, Boston and San Francisco -- not just at Metro."

King said it was his position that the Urban Mass Transportation Administration, which funnels federal grants to transit authorities, should vigorously assume responsibility for establishing safety standards.