One bad train spoiled the lot for Metro subway commuters yesterday morning as first-week problems continued to foul up the operating schedule on the new Blue Line from National Airport to RFK Stadium.

Metro was forced to run the morning rush with seven trains instead of 12. That meant that trains were running every 15 minutes instead of every seven - the interval Metro would like to have during rush hour on the Blue Line.

As a result, crowds stacked on station platforms, then had to squeeze onto already overfull trains.

The problem is the configuration of the National Airport terminal.All trains must pass over an electronic safety test loop, and pass the test, before they leave the Airport each morning. There is only one test loop.

Because of inadequate switching facilities at the airport, there is no place to put a bad train that fails the test while good trains are passed around it. The third train out failed yesterday and it was 9 a.m. before Metro could get everything cleared up and a full complement of trains on the track.

Metro General Manager Theodore Lutz told the Transit Authority Board yesterday that additional safety test loops are being installed at the Airport to expedite the tests.

One train breakdown was reported at Arlington Cemetery during the evening rush hour.