The National Transportation Safety Board and the Federal Railway Administration both started formal investigations yesterday of the collision and derailment of two Metro trains making test runs late Sunday night.

Both agencies customarily make preliminary inquiries after transit accidents. Their formal entry into this case indicates they regard it as unsual or serious. "We want to make sure that every avenue is plugged for something like this to happen in regular train operations," a safety board spokesman said.

Ten employees of Metro or its contractors - the only people aboard the two trains - were slightly injured, Metro said. One Metro employee, Vince McGowan, 53, was hospitalized for treatment of a possible heart attack and was reported in fair condition yesterday at the Washington Hospital Center.

Metro itself was setting up an official board of inquiry. Metro General Manager Theodore Lutz declined comment on the incident until the board completes its investigation.

Although the accident deprived Metro of an important section of track, there was little apparent effect on subway schedules yesterday. Metro officials said both the morning and evening rush hours went smoothly.

The investigation, according to Metro officials interviewed yesterday, is concentrating on the operating instructions that were given to the train crews and the amount of caution the crews exercised when they took their trains into an X-shaped crossover switch, where the accident occurred.

It appears from early investigation that both trains went through red signals, in effect running red lights. "We've got to find out what their instructions were," said Ralph Wood, Metro's director of train operations. "They may have been told not to obey the reds."

According to wood and others interviewed, both trains had been conducting tests on the unopened segment of Metro's Red Line between Rhode Island Avenue and Silver Spring. That section is scheduled to open Feb. 6.

Both trains were traveling south on parallel tracks towards Rhode Island Avenue. The train in the easternmost track - the normal northbound track - was the first to approach the crossover that connests the two tracks.

After entering the switch area, the train stopped. The other train entered the switch from the other track and was carried into the right front of the first train.

The two trains bounced along the tracks for a ways and came to rest in the middle of the switch. Both of them were derailed. Each train was only two cars long.

The first train, the one that stopped, was operated by Richard Jackson, 31, according to Metro spokesmen. The second train was operated by James Coan, 36. Both men have been train operators since April 4. Normally, new operators are assigned work on test trains befor operating on passenger run, Wood said.

Both trains were being operated manually. That means that power settings and brake applications were made by the operators themselves instead of by the computer that controls Metro trains during passenger service.

Furthermore, the "automatic train protection" system that provides a further safeguard to passenger carrying trains was not in operation. That supersensitive system, when turned on, has occasionally been the reason that Metro trains have refused to budge in normal operations.

Test runs, such as those being made Sunday night, are required to check out switches, signals and computer commands to the trains.

Each train carried two operators and one supervisor, all Metro employees. A total of four exployees of Metro contractors responsible for the overall construction of the subway system and for its sophisticated electronic signal equipment also were on board.

Joseph Greenway, who is in charge of the trackside signals and the electronic information that controls normal trains confirmed that signals for both tracks were red when the two trains approached the crossover.

When asked what happened, Greenway said, "They didn't check their iron." Translated, that means that the operators did not make a visual check of the switch alignment to see if their trains could safely proceed.

If the automatic train control system had been truned on, the train would refuse to move through the red signal.

Automatic train protection can be overridden during passenger operations, but the train operator must be given permission to do so by central control. Then, a 15 miles and hour speed limit is imposed on the tran and it carries passengers only as far as the next station.

Metor's procedures during normal operations will get the emphasis of the federal investigations. Donald W. Bennett, the Federal Railway Administration's associate administrator for safety, said, "We want to find out what happened, and then see if there is any condition we should make a recommendation about to Metro."

The accident itself occurred on the section of track between Rhode Island Avenue and Franklin Street NE. The crossover involved has been used since Metro started subway service in March, 1976, to change trains from northbound to southbound tracks.

Some power cables to the switch were cut during the accident and the switching mechanism probably was fouled, Wood said.

The loss of that crossover meant that Metro had to transfer trains between tracks on another crossover between Rhode Island Avenue and Union Station Metro stations.

That meant trains at the Rhode Island Avenue station sometimes came and went on the "wrong" platform. But Metro personnel made repeated announcements about whatwas happening during the morning rush hour and there was a minimum of confusion.