An old landmark -- or perhaps we should say an old watermark--of Washington has disappeared. The shanty that was occupied for many decades by the bridge tender on the old Long Bridge, the railroad equivalent of the 14th Street highway bridge across the Potomac River, has been dismantled.

The razing occurred several weeks ago, according to a spokesman for Consolidated Rail Corp. (Conrail), which now owns the span. But the disappearance came into MetroScene's sight only a few days ago during a drive across the highway bridge.

The shanty, apparently of corrugated metal, was perched atop the section of the bridge that pivoted to permit boats--mostly commercial ones--to pass through on their way to and from Georgetown.

When the "new" southbound highway bridge, officially named but rarely called the George Mason Bridge, was opened in the early 1960s, it had no draw span, thus blocking the river to all boats but those with low silhouettes. Since then, no tender had been stationed in the railroad bridge's shanty, and the structure became useful only as a billboard for graffiti, most memorably "Beat St. John's" or "Beat Gonzaga."

"If the bridge doesn't open, there's no need for a bridge tender's shanty," a Conrail spokesman said yesterday.