Metro officials have several options but no simple or cheap solutions to the problem of minimizing the risk of freight train derailments along corridors shared by parallel Metro and railroad tracks, according to rail safety experts.

"Not a lot can be done," said John H. Riley, chief of the Federal Railroad Administration. "One can legitimately question the judgment of laying Metro along railroad track so closely . . . . I don't know how to fix it where it is now."

Costly options such as tunneling or elevating Metro tracks, widening the shared corridor, building barriers between tracks or limiting freight train operations will have to be measured against the risks, officials say.

Twice in the last three months -- on June 19 and again Sept. 5 -- CSX freight trains have partially derailed from tracks that parallel the Red Line between Union Station and Silver Spring. Both times they ripped up hundreds of feet of Metro track and interrupted Metro service to the Takoma and Silver Spring stations for at least three days.

The wrecks caused no injuries, but the specter of a fast freight plowing into a fully loaded rush-hour Metro train has prompted a National Transportation Safety Board special investigation and a congressional hearing to examine the dangers and issues raised.

More than 20 miles of railroad tracks run parallel to Metro along parts of the Red, Orange and Yellow lines. An additional 8.5 miles of Metro runs down the median of a major freeway, I-66 in Northern Virginia. Metro's construction plans include laying another 11 miles of the Green and Yellow lines alongside existing railroad track and 2.1 miles along the Suitland Parkway.

"The decisions made 20 years ago are simply a fact of life," Metro General Manager Carmen E. Turner said of the transit agency's decision to build as much of the system as possible alongside railroads and highways. "The issue is how do we operate with the utmost safety in that corridor."

Options being considered or proposed by Metro, CSX, the safety board and others include: Putting the Metro tracks in a tunnel or elevating them above CSX tracks. That would eliminate the danger, but at a cost that would be financially and politically prohibitive. One reason Metro chose to lay track alongside railroads in the first place was to reduce construction costs. Metro construction on the surface currently costs $11 million to $13 million a mile, while tunneling can cost $28 million to $68 million a mile depending on the type of earth. An aerial track, such as at National Airport, costs more than $25 million a mile. Widening the corridor. The Sept. 5 derailment occurred at a spot where Metro and CSX tracks lie 20 feet apart, measured from center to center. The Metropolitan Atlanta Rapid Transit Authority (MARTA), built after Metro, generally requires a 50-foot separation in shared corridors. Nonetheless, a CSX train derailment last year threw two 75-foot-long freight cars across MARTA's tracks. Many freight cars are 100 feet long. Buying the land in Washington's real estate market to allow 100 feet between tracks would be prohibitively expensive, many observers say. Building barriers or separating the tracks by height. In the areas where the separation is less than 50 feet, MARTA has built its line 10 feet higher than the railroad, on a mountain of earth secured by a 1-foot-thick concrete retaining wall. MARTA also has erected concrete barriers around columns that support elevated track. While barriers can deflect the impact of a crash, no feasible wall could be built that could stop a speeding derailed freight car. "The loads are so tremendous, you couldn't build anything that could withstand a direct hit," said John Brach, MARTA's director of engineering. Restricting speeds. Slowing trains reduces the impact of a crash. However, trains have a higher chance of derailing at speeds below about 25 mph, because the cars start rocking, according to railroad experts. CSX has slowed its trains alongside the Red Line from Silver Spring to Union Station to 30 mph from 55 mph while a CSX-Metro committee studies the common corridor issues. However, railroad industry experts say a permanent speed restriction would hurt CSX economically in its competition for freight traffic. Shortening trains. CSX has already rejected suggestions to shorten its one- to two-mile-long trains, arguing that length does not cause derailments. However, railroad experts say that the longer the train, the higher the forces pushing the cars together and stretching them apart. This accordion movement puts more pressure on couplers and track and is particularly acute in hilly territory such as that along the CSX line west of Washington. "If you have a weak link, {a longer train} will find it," said one railroad safety specialist, who asked not to be identified.

The June 19 derailment near Takoma Park has been blamed on a cracked center beam in a refrigerator car. "Train dynamics found the weakest link," the specialist said.

Despite the problems with long trains, shorter trains mean more trains, and that too increases the potential danger to Metro, CSX officials say.

Changing schedules. CSX rejected as impossible suggestions that it operate freight trains only when Metro is not running, from midnight to 6 a.m. Fifteen to 20 freight trains pass through the corridor a day, depending on shippers' needs and the time of year. Safety experts argue against any limitations on Metro's operations, saying that riders are safer on Metro than on highways.

Building stronger Metro cars. While many rapid-transit systems are built alongside railroad tracks, Metro is a source of more concern because of its lightweight aluminum cars, safety experts said. In 1982, a Metro car backing up at less than 10 mph hit a barrier, killing three passengers and injuring 25. A safety board report said the wall of the car "was easily severed and there was little remaining structural integrity."

The report noted that the safety board in 1970 had "expressed grave concern over the design and structural integrity" of Metro's cars.

Metro studied the issue, but "it was not deemed feasible" to order new cars that would be heavy enough to withstand a serious impact, yet light enough to operate on Metro's system, said Fady P. Bassily, assistant general manager for rail service. There are about 600 cars in the Metro fleet, all of essentially the same design. Regulating. No federal agency sets rules or guidelines for the construction of rapid transit systems along side railroad tracks. However, the Federal Highway Administration requires barriers, grade separation or other measures when a transit system lays track along a federally funded highway, such as I-66. Nonetheless, in November 1983, before the parallel Orange Line track was open for passengers, an automobile on I-66 crashed over the barrier and onto the Metro tracks. Metro then doubled the height of the barriers, to 64 inches. Improving Metro's warning system. A fence between Metro and parallel railroad tracks is wired to signal Metro headquarters if an "intruder," such as a freight car, crosses the boundary. A person in Metro's central operations control center must see the signal and then cut power to stop Metro trains. The system worked in both recent derailments, but some safety specialists have suggested that it be automated to cut Metro power instantly without requiring human intervention.

Rep. Thomas A. Luken (D-Ohio), chairman of the House Energy and Commerce subcommittee on transportation, tourism and hazardous materials, said his staff is drafting legislation to establish safety rules in joint rail corridors.

Riley, of the railroad administration, said last week that regulation is "something I'll look at."

Alfred A. DelliBovi, chief of the Urban Mass Transportation Administration, which grants money to rapid transit projects, said no general standard would suit all transit systems.

UMTA monitors the safety of construction through independent consultants hired to review project managment, DelliBovi said. UMTA will direct the firm that reviews Metro construction to review plans for the shared corridors on the Green and Yellow lines, he said.

Despite the risks of derailment, many transit systems have found advantages to building their lines alongside railroads, said MARTA's Brach. "That is a transportation corridor well suited for rail, and it's not disturbing neighborhoods," Brach said. "We try to be near railroads, but there are trade-offs."