Metro General Manager Carmen E. Turner said yesterday that she will ask CSX Corp. officials to consider immediate changes in freight train operations to prevent another derailment alongside Metro tracks like the Saturday night wreck that is expected to cut off Red Line service north of the Fort Totten station at least through tomorrow morning.

Turner said she will ask to meet with CSX officials to discuss altering the railroad's freight train lengths, schedules and speeds through the corridor between Rhode Island Avenue and Silver Spring, where Metro and CSX trains run on parallel tracks. The CSX derailment was the second in that corridor in three months.

"We want to discuss a whole range of operational areas . . . to at least reduce the probability of this happening again," Turner said.

The transit agency plans today, for the third day in a row, to provide free bus service between the Fort Totten, Takoma and Silver Spring stations. Commuters using those stations should allow an extra half-hour for their trips, Metro officials warned.

CSX workers continued to clear wreckage at the derailment site near New Hampshire Avenue north of the Fort Totten station in Northeast Washington, and Metro workers started repairs on more than 1,000 feet of track. Transit officials said they hoped to resume full service by tomorrow's evening rush hour.

Commuters also can ride today between Silver Spring and Union Station on six rush-hour Maryland Rail Commuter Service, or MARC, trains, a MARC spokesman said. The commuter rail line plans to operate trains on CSX tracks that were repaired after the derailment. MARC trains are expected to encounter 10-minute delays as they travel through the derailment site, said spokesman Bill McCaffrey.

On a typical weekday morning, about 9,500 Metro riders board at the Silver Spring station, the busiest in Maryland. About 3,500 passengers get on at Takoma during the morning rush hour. Over an entire weekday, an average of 19,300 passengers board at the two stations.

The 11:23 p.m. derailment of 12 freight cars occurred less than 2 miles south of the derailment at 4:40 a.m. June 19 of 21 cars on a CSX train.

No one was injured in either incident, but hundreds of feet of Metro track were damaged within sight of Metro stations. The two derailments raised fears of a possible rush-hour catastrophe and triggered calls from local officials and a federal safety expert for CSX to restrict its freight trains through the area to hours when Metro trains are not running.

"At an absolute minimum, they should separate schedules tomorrow," said Maryland Del. Peter Franchot (D-Montgomery County), a Takoma Park resident who criticized Metro and CSX in a recent hearing for failing to take preventive action after the June derailment. "They've got to be put on notice that it's not business as usual, they can't just clean this up again and act as if it never happened."

The proposal to impose restrictions on CSX's operating hours was endorsed by Metro board Chairman Joseph Alexander, a Fairfax County supervisor; Metro safety committee Chairman Carlton R. Sickles, and a railroad safety expert, who asked not to be identified.

"It sounds like a good idea to me," Sickles said.

As an interim measure, Turner said she wants to discuss possible changes, dealing with "scheduling primarily," to serve until the railroad and the transit agency can work out a "permanent solution" to reduce hazards.

R. Lindsay Leckie, a CSX spokesman, said that CSX officials would be "happy to listen to what they have to say." But he added that it would be "a very difficult imposition on CSX" to limit its freight train operations through the area to hours when Metro is not running, chiefly from midnight to 6 a.m.

"There is more to this than looking at limiting our window of operations on property that has been ours for more than 100 years," Leckie said. CSX, based in Richmond, was formed in 1985 by the merger of the Chessie System -- a combination of the old Baltimore & Ohio and Chesapeake & Ohio lines -- with the Seaboard Coastline Inc. rail.

"We will look at any proposal put to us by Metro. Our response remains to be seen," Leckie said.

Along the stretch from Rhode Island to Silver Spring, CSX operates about 14 freight trains a day. Two Amtrak trains and six MARC trains also operate on the CSX tracks. Metro runs trains through the area every three minutes during rush hours, every seven minutes at mid-day and about every 12 minutes in the late evening.

At the time of the Saturday derailment, the nearest Metro train was close to the Takoma station, heading south on its way to a yard with only an operator on board. Another train, carrying passengers, was near the Brookland station, heading north toward the derailment site. Both stopped immediately because the derailment triggered Metro's warning system and cut off power in the area, Metro spokeswoman Beverly Silverberg said.

Metro trains operate at speeds up to 59 mph through the area. The freight train pulled by four locomotives was traveling at 55 mph, within the speed limit for the corridor.

Turner said she will ask the National Transportation Safety Board to speed up an investigation it launched after the June accident. The safety board probe is designed to examine dangers posed to Metro by operating alongside freight trains.

A separate Metro investigation of the June accident will be broadened to include a review of possible hazards stemming from freight train operations beside Metro tracks "throughout the entire system" and along Metro routes scheduled to open in the future, Turner said.

Of Metro's 69.6-mile system, more than 20 miles on the Yellow, Orange and Red lines run alongside railroad tracks. Another eight miles are in the median of I-66 in Northern Virginia. In the past, similar safety issues were raised because of concern over possible collisions between Metro trains and highway vehicles. On average, Metro tracks and parallel railroad tracks are about 35 feet apart.

Metro plans two extensions alongside railroad tracks. These are a proposed Green Line section between College Park and Greenbelt in Prince George's County, and a Yellow Line extension to a Franconia-Springfield station in Fairfax.

The federal safety board warned Metro planners in 1970 that sharing corridors with railroad tracks would create "potential hazards" in the event of a derailment. Metro chose to lay as much track as possible along railroads and highways to reduce construction costs and minimize disruptions to neighborhoods.

Since then, three freight train derailments have occurred in the Red Line corridor, including the two this year. In July 1976, two years before this section of the Red Line opened, 20 cars on a Chessie System train derailed at Blair Road and Underwood Street NW, a site between the locations of the two recent accidents.

In November 1983, an automobile traveling along I-66 flipped over a 32-inch concrete barrier and landed on both inbound and outbound tracks of the Orange Line, which had not yet opened for service. Metro later doubled the height of barriers along the median as a safety measure.

Safety board investigators seeking to determine the cause of Saturday's accident continued to focus on apparent wheel problems in a freight car that was the first to derail, along with a broken track switch at the site where the derailment apparently began.

CSX officials said yesterday that they had not determined the cause. The June 19 derailment was attributed to a break in the center beam of a refrigerator car and a chain reaction triggered by the uneven application of the automatic brakes.

According to the safety board's intitial investigation, the Saturday accident apparently began almost half a mile north of the site where cars numbered 45 through 56 came uncoupled from the front of the train. The cars crashed into one another, spilling across two Metro tracks and another CSX track and plowing into a nearby warehouse and Metro power station.

"Evidence in the track structure" indicates that a wheel on the left side of car number 45 first left its proper position on the rail at the site of a broken switch point, said William G. Meeker, a safety specialist leading the investigation. A switch point is an end of a track that can be switched to either of two connecting tracks.

About seven inches of the point is broken off. Metallurgical tests will be conducted to determine if it broke before or after the derailment, he said.

Railroad wheels normally hook under a protruding rail lip. In this case, Meeker said, when the southbound train passed over the switch point, the wheel hook, or flange, of car 45 slipped out from under the lip so that the wheel was riding along the top of the rail for several hundred feet before falling off to the outside of the rail.

The car remained coupled to the train until the train started rounding a curve, Meeker said. At the point of the curve, just south of New Hampshire Avenue NE, car 45 became disconnected from cars in front of it, triggering the automatic brakes.

The four locomotives and 44 cars continued another mile before coming to a stop. The cars behind number 45 derailed, crashing into each other. Cars 57 through 90 stopped without derailing.

The first derailed car traveled almost half a mile without hitting the warning fence between railroad tracks and Metro tracks, Meeker said. When hit, the electronic system sends a signal to Metro's operations center to shut down power in the immediate area. The system was triggered at the point of the 12-car pileup, just north of the Fort Totten station.