We're up on another three-day weekend, during which thousands of people will arrive back in town at Union Station, National Airport and the bus stations on Monday evening only to find that our usually marvelous Metrorail system has gone to sleep early. In bush-league fashion, Metro shuts down at 6 p.m. on such nights when it's most needed.

We inquired anonymously of Metro information how we might get by bus to two hypothetical destinations Monday night from the train station and the airport. The answers:

* If you arrive at National on an airplane at 9 p.m. and want to go to Ballston, a scant four miles away in North Arlington, you have a choice of a bus to Alexandria, transferring to another bus that meanders across Arlington, getting you to Ballston (depending upon transfer connections) at either 10:23 or 11:23 p.m., for a fare of 75 cents. Or you can go by bus into Washington, make two transfers (at the Federal Triangle and in Georgetown), arriving at Ballston at 10:56 p.m. for a fare of $2.25. Yes, an extortionate $2.25, because you have to pay Metro for your inconvenience of making two river crossings.

* If you arrive at Union Station on the 9:03 p.m. train from New York and want to go to Silver Spring, you must take a bus to downtown Washington and transfer to the 16th Street line, arriving at your destination at 10:12 or 10:42 p.m., depending upon transfers. The fare: $1.10.

If the subway were running, the Airport-Ballston trip would take 25 minutes instead of roughly two hours. The Union Station-Silver Spring trip by train would take 14 minutes instead of 1 1/2 hours. And both for 75 cents.