Federal safety officials cautioned in 1970 that Metro's plans for building parts of its transit system along existing railroad tracks would create "potential hazards" in the event of a derailment.

The transit agency was reminded of that 17-year-old warning yesterday when 21 freight cars on a CSX Corp. train slipped off the rails and ripped up 800 feet of parallel Metro track just north of the Takoma station on the Red Line.

Occurring at an hour when Metro trains were still, the 4:40 a.m. accident caused no injuries. But yesterday's accident raised the spectre of the rush-hour catastrophe that would have occurred had the freight cars plowed through a fully loaded Metro train.

That potential hazard was debated in the 1960s, when Metro's planners chose to lay as much track as possible along railroads and highways to reduce construction costs and minimize neighborhood disruptions. At the same time, they took what steps they could to reduce safety risks, officials said yesterday.

"The distance between the Metro tracks and the railroad tracks will not be sufficient to prevent interference in cases of derailment," the National Transportation Safety Board wrote in an October 1970 report listing "potentially serious identifiable hazards" in Metro's construction plans.

"There are no plans for means or devices to prevent excursion of Metro trains or conventional trains from their tracks in cases of derailment," the board wrote.

The board also cited the danger of locating Metro tracks along public highways. "The proposed highway guard rails and chain link fencing will not prevent a loaded truck from encroaching upon the Metro tracks," the board wrote.

Metro's general manager at the time, Jackson Graham, answered that the board's "purported 'safety problems' are indictments not of {Metro} but of the entire rapid rail transit industry: in many respects, of all surface transportation across the country." He said Metro would study the report, and vowed that its system would "be second to none anywhere in safety of operation."

Since then, three accidents including yesterday's have illustrated the board's concerns, while continuing improvements in safety measures and devices reflect Graham's promise.In July 1976, 20 cars on a Chessie System freight train derailed at Blair Road and Underwood Street NW at a site that had been cleared for installation of track for the Red Line, which opened two years later. That derailment was not far from where yesterday's incident occurred. The Chessie System has since been incorporated into CSX. In November 1983, an automobile being driven along I-66 in Virginia flipped over a 32-inch concrete barrier and landed on both the inbound and outbound tracks of the Orange Line, which had not yet opened for service. Metro doubled the height of the barriers.

There are 69.6 Metro miles in operation, 37 of them outdoors. Of those, 20 run alongside railroad tracks and eight run in the I-66 median.

"It's a fixed situation at this point," Metro board member Cleatus E. Barnett said. "There is no feasible way to change the circumstance at this point."

Earlier this week, the Metro board voted to increase its liability coverage at the request of CSX, which cited the Conrail-Amtrak collision north of Baltimore in January that killed 16 people as an example of the risks involved, Barnett said.

The advantages of building Metro in existing transportation corridors are obvious: Construction above ground is far cheaper than tunneling and avoids the need to cut new routes through residential areas. "It's often the only way to get an above ground right-of-way," Barnett said.

To counter the attendant risk, Metro has taken some unusual safety precautions.

The agency planned as early as 1970 to allow at least 20 feet between the center of Metro tracks and the center of adjacent conventional railroad tracks, which is four to eight feet more than the rail industry standard. In fact, the average distance between Metro's tracks and parallel tracks is 35 feet.

Metro worked with railroad industry specialists in 1975 to develop methods to reduce the possibility of a derailment in the shared corridors. Hot lines were established, fences were built to separate Metro tracks from CSX, and sensors were installed along the tracks to detect flaws in the rails and to determine if objects were being dragged under a train, a potential cause of derailment, one member of the working group recalled.

Metro also installed an electronic "intruder alarm system" along the fence separating Metro tracks from railroad tracks, to alert officials of any trouble.

The system worked perfectly yesterday, said Roger Wood, Metro's safety director. About 4:40 a.m., when the CSX cars tore the fence between the tracks, an electronic signal immediately notified Metro's central operations control center of an "intrusion" and defined its location and length.

The control center supervisor shut down third-rail electrical power along the inbound and outbound Metro tracks in the immediate vicinity to prevent trains from passing through, said spokeswoman Beverly Silverberg.

The supervisor notified the nearest CSX railroad tower of a problem in the area, so no freight trains would be allowed into the area.

Metro's first safety worker was on the scene within seven minutes, Wood said. "It all worked as it is supposed to," he said.