Startled subway passengers watched police with guns drawn shoulder their way through a crowded downtown train in pursuit of two bank robbers at midday yesterday, thus becoming witnesses to Washington's latest phenomenon - the Metro get-away car.

The robbers held up a branch office of the National Bank of Washington at 15th and I Streets NW. and then used the subway as an escape route, fleeing into the McPherson Square Metro station one block away.

Ten minutes and 3 1/2 miles later. D.C. Police Sgt. Richard R. Henry spotted two males fitting the description of the robbers as they walked in the 1100 block of D Street SE near the Eastern Market Metro station.

Henry arrested one, a 15-year-old-youth with $114 stuffed inside his shirt. The other youth escaped.

Police said the 15-year-old is being questioned in connection with the robbery of six other banks since June. In most cases, they said, the suspects made their getaway on the subway.

With expansion of the new subway system, said Det. Sgt. Jimmy King of the D.C. police bank robbery squad, the city can expect an increase in Metro getaways.

The speed of subway trains and the ability of suspects to get lost in the crowds, police said, are the principal incentives.

In yesterday's getaway, a Metro attendant at the McPherson Square station said uniformed police poured into the station and escaped over the fare gate to a waiting train.

"I guess the suspects were ahead of them, but I couldn't tell. They must have had Farcards like every body else," the attendant said.

In addition to the 15-year-old arrested in Southeast, police detained two other suspects at the Foggy Bottom Metro station but released them after witnesses at the bank failed to identify them.

As D.C. police pursued the suspects on trains going in two different directions from the McPherson Square station, transit police relayed a description of the suspects to FAA police at National Airport and Arlington County police where Metro also now goes - a routine procedure for Metro's increasingly complex interjurisdictional status.