The number of passengers riding Metro subway trains shot up last week to a record level of nearly 470,000 a day, outstripping Metrobus ridership for the first time since the rail system opened 10 years ago, Metro officials said.

The new statistics appear to mark a milestone for the transit authority, officials said. The bus system, once the mainstay of Washington's mass transit network, has increasingly become a "feeder" service for the expanding 60.5-mile subway system, they said.

Subway ridership last week averaged 469,912 trips a day, surpassing the previous record of 439,639 set last July. Last week's rail patronage sharply exceeded the 400,426 level reached in April 1985.

Bus ridership rose more moderately, officials said. Passengers took an average of 463,895 trips a day last week, compared with 447,950 in April 1985.

Officials attributed the ridership gains to increased employment, economic growth and tourism, along with the rail system's expansion.

Subway patronage has climbed markedly on the Red Line extension to the Shady Grove terminus near Gaithersburg, which opened in December 1984.

Metro ridership had declined during the early 1980s because of more plentiful gasoline, high unemployment, fare increases and frequent breakdowns by subway trains and buses. The trend was reversed in 1983. Since then, the authority has taken steps to improve service and hold the line on fares.

Metro officials cited the new ridership statistics in an attempt to counter a controversial estimate in a recent study by the Federal City Council, a nonprofit group. The study predicted that Metrorail ridership would average only 469,000 trips a day by 1993. Last week's total slightly exceeded that level, Metro officials pointed out.