As it has a habit of doing, the truth has finally leaked out. But for a few days last month, the only thing the regulars aboard Maryland Rail Commuter (MARC) trains could talk about was . . . .

The Memo.

It looked as official as could be -- what with the Amtrak insignia across the bottom and the word NOTICE across the top. Here's what it said:

"Amtrak and Chessie System railroad police officials have received numerous complaints over the past several weeks from motorists who have been 'mooned' while driving on roadways adjacent to Amtrak and B&O rail lines by passengers on board MARC commuter trains.

"Passengers are hereby notified that 'mooning' is a violation of Maryland and D.C. laws, as well as Amtrak and MARC regulations.

"Effective immediately, plainclothes railroad police will be riding all MARC commuter trains in an all-out effort to halt this obscene practice. Any passenger caught in the act of 'mooning' will be subject to arrest."

Well! MARC regulars knew they were a pretty friendly bunch. After all, birthdays are seldom forgotten on the MARC lines between Baltimore and Brunswick, Md., and Washington. Romances occasionally blossom. So do job prospects. And bottles of strange and wonderful liquids have a way of becoming uncorked on Friday nights.

But exposing one's posterior to passing motorists? None of the regulars had ever seen it happen, or heard of it happening. Was The Memo for real?

Betty Fitch, MARC's secretary for passenger services, looked into the matter. She discovered that The Memo was the product of two things: someone's copying machine, and someone's imagination.

Who was the someone? His identity remains unknown, but Betty figures him for a regular who was looking for a few yuks. Heaven knows, he succeeded.

Have mooning and MARC ever actually crossed paths? Yes, indeed. But the moon was aimed at a train, not from it.

According to Kathy Waters, a MARC passenger services specialist, a man owns some land near the MARC station at Point of Rocks, Md. Passengers are always walking or driving on his property. This upsets the man considerably. One day, according to Kathy, he got so upset that he mooned a trainful of passengers who had just trespassed.

So, if you've ever wondered what they mean by the expression, "Hell of a way to run a railroad," I trust you're wondering no more.

And while we're on the sensitive subject of That Upon Which We Sit . . . .

Buffalo, N.Y., is seldom considered a trend-setting metropolis. But the American Lung Association of Western New York, which is based in Buffalo, has had great success with a new stop-smoking ad campaign.

The centerpiece of the campaign is this slogan:

Kiss Your Butt Goodbye.

The question before the house:

Is the slogan too risque for dear old Washington, D.C.?

Kiss Your Butt Goodbye is an unusually ingenious ad campaign, because the slogan works in two directions at once. If you stop smoking, you are kissing your butt goodbye in one sense. If you continue to smoke, you're kissing it goodbye in another sense.

ALAWNY was aware that some people might find the campaign offensive. So executive director Margie Turner "market-tested three groups: people in a shopping mall, personnel directors and the media. Everyone loved it."

Most people have felt the same way in Minneapolis, Los Angeles, Dallas, Oregon and New Mexico, where the campaign is now running, too. And Washington may be next. But the communications director of the D.C. Lung Association is a leeeeeeetle nervous.

"We may be doing it," said Marisa Sandifer. "But at this point, it is still unsure. The youth like it, but Washington is conservative."

How about it, Washingtonians? Are we so conservative that our sensibilities would be offended by the notion of kissing butts goodbye? Are we so stuffy that we can't live with a public pun? Are we so uptight that any mention of our puffy posteriors, even in jest, becomes too much to bear?

I say River City can handle it.

I say that beneath all those button-down collars and those mountains of telephone message pads, Washington has a sense of humor.

I say that Babylon-by-the-Potomac can live with the idea of kisses, butts and goodbyes -- jointly as well as separately.

Anybody agree? Or think otherwise? Drop me a line in care of The Washington Post, Washington, D.C., 20071. I'll publish the results in a few days.