A Metro subway train broke down between stations under downtown Washington yesterday morning and its passengers had to be transferred to another train in the middle of the tunnel.

The four-car train was between Metro Center and McPherson Square on the Blue Line when its emergency brakes were automatically activated, a Metro spokesperson said.

Metro did not know how many people were on the train, but at 8:53 a.m., when the incident occurred, a typical four-car train would carry 400 to 600 people.

Train operator B.D. Tracy, the man who operated the first train on both of Metro's formal openinnnngs, tried for 17 minutes to get the brakes to unlock, but failed.

Another train, which was in the Metro Center station, was unloaded, then brought up in the tunnel immediately behind the crippled train. People were transferred through the bulkhead doors. The rescue train backcked into Metro Center.

Rescue trains do not couple with stricken trains because "we have found that if we couple one good train to a bad train, we get a longer bad train - the problem just transfers through the electronic connections," a Metro official said.

here is about a four-inch gap between trains that the passengers have to cross in such an operation. It is at least the fourth time Metro has had to run a rescue train since it began operations here in March, 1976.

The entire operation yesterday lasted almost an hour. While the tunnel was blocked, the parallel track was used for two-way operations - thus greatly increasing the normal 10-minute interval between trains in each direction.

After all passengers were evacuated, electronic safety locks were cut out, the track was cleared and the train was pushed to National Airport. Metro was investigating to determine what caused the breakdown.

A similar event occurred during the evening rush yesterday when the automatic brakes were applied just as a train pulled into the Arlington Cemetery station. Three of the four cars were at the platform, however, and passengers from the fourth car just had to walk one car forward to get out.

Unnecessary automatic braking has been the second-most common failing of the $300,000 Rohr Industries Metro cars, trailing behind sticking doors on loaded trains that prevent trains from moving.

Rohr Industries said yesterday that it is sending a team of top-level technicians to Washington on Monday to discuss the operational difficulties Metro has hadwith the Rohr cars since the Blue Line between National Airport and RFK Stadium opened on July 1.