Most summer days, their shrieks and cannonball splashes reach far around the bend in the C&O trail, a half dozen youngsters hurling themselves off the old railroad trestle at Arizona Avenue NW into the canal 25 feet below.

Swimming in the canal is forbidden. And the National Park Service recently strung a cable across the entrance to the trestle to keep people off the newly planked, 75-year-old crossing that many hope will one day be a part of the Capital Crescent bike and hiking trail.

But the cable does little to keep out weekend strollers who just step over it, as do youngsters from the neighborhood who use the trestle as access to their own private swimming hole.

"It's fun, it's refreshing and it should be legalized," said a 13-year-old boy in a dripping T-shirt and neoprene shoes.

"It's an attractive nuisance," said Earl Kittleman, spokesman for the National Park Service, which has jurisdiction over the trestle. "We're really in a liability situation."

Last week, the 100-foot bridge was the subject of deliberations on Capitol Hill, where the House Interior appropriations subcommittee approved about $20,000 for the Park Service to put up a 12-foot-high chain-link fence on each side.

It would mean the end of an era.

The trestle is part of a railroad spur built in 1915 to haul coal from Silver Spring to a Georgetown electric plant run by the federal government. Since then, generations of children have used it as a meeting place, diving platform and blackboard for their thoughts.

The Park Service took jurisdiction over the trestle last year when it leased the four-mile District leg of the abandoned rail bed after it helped Montgomery County buy the seven-mile Maryland leg in 1988.

The plan is to clear the tangle of wild grapes and trumpet vines that obscure the rail bed and turn the ribbon of land into an 11-mile linear park. The park would include a bicycle trail that advocates estimate 1 million people a year would use for recreation and to pedal to work.

Completion of the Maryland leg is scheduled for 1994. But how soon the District leg is open depends on Congress. A proposal to provide $7 million for the Park Service to purchase the property was approved by the House panel last week, but could meet resistance in the Senate.

"We've got to have off-road routes like this if bicycling is to become a serious commuting alternative," said Tom Pendleton, the District's bicycle coordinator.

A coalition of more than 20 bicycle, hiking and environmental organizations has pressed for construction of the trail since CSX Corp. put the rail bed up for sale two years ago.

But the Park Service's move to close the trestle angered coalition members. They thought their campaign was making progress when the Park Service leased the land last year and said local groups could plank the trestle for easier walking.

Members of local conservation groups and coalition volunteers worked side by side to cover the rail ties with heavy wood planks last spring. They also hung barrier cables along the open sides. But vandals cut those, and the Park Service closed the trestle.

Coalition members maintain that the trestle is no less safe now than it was during the 75 years since it was built.

Park officials insist that the trestle is unsafe but that the agency can't put up effective safety barriers until it owns the property. They say no injuries have been reported since the service began overseeing the trestle, but they do get complaints about youngsters jumping off it.

In the meantime, the trestle, now a neighborhood institution, hardly seems off-limits to anyone. Fresh graffiti appears after each weekend. And youths continue to make the jump.

"I've been coming here since I was a kid and I'm not stopping now," said one swimmer. "Besides, who are they kidding with those little cables?"