An engineer could have missed a red light. Blowing snow could have packed into a mechanical switch.
Investigators are considering these and other possibilities as they search for whatever turned an apparently routine train maneuver into a collision between an Amtrak train and a MARC train that left 11 dead.
But one safety advocate contends that even if one of several most-likely mistakes had occurred, the system should have been able to catch it in time to save lives.
Vulnerability of passengers
Lawrence M. Mann, a railroad safety attorney who works with unions and helped write the Federal Railroad Safety Act of 1970, said the Amtrak train should have been able to stop, that passenger cars should be stronger and that the MARC riders were made too vulnerable by being seated in passenger cars pushed by a locomotive.
Mr. Mann said he and others have been trying to persuade Congress and federal agencies to make train travel safer.
Two good years for safety
Federal Railroad Administration officials said safety is a top priority, pointing to statistics showing that that 1994 was the safest year to date in rail history. Preliminary figures indicate that 1995 may have been even safer.
Yesterday, officials with Amtrak, MARC and CSX Transportation Inc. -- the company that owns and operates the tracks -- said they can't comment on details until the National Transportation Safety Board completes its investigation.
What is clear is that on the day of the crash, CSX inspectors had checked the switch and signal on the stretch of snowy track in Silver Spring where the trains would collide and found no problems.
CSX officials also said the very same situation -- with one train passing another while a third waited -- had occurred at least once earlier that afternoon, and the switches and signals functioned properly.
Marty Fiorentino, spokesman for CSX, said the company believes that the same switches and signals worked correctly at the time of the accident.
Using computer technology and telecommunications, dispatchers at CSX's transportation center in Jacksonville, Fla., actually operate the signal lights and move the switches for an average of about 1,300 trains on 18,000 miles of track through 20 states -- including the tracks in Silver Spring.
150 miles per dispatcher
Although the workload varies by region, a dispatcher working the Northeast region typically controls about 150 miles of track carrying about 19 commuter trains a day, and a similar number of freight trains.
Mr. Mann said the number of dispatchers has been cut by about 50 percent over the last 10 years, but CSX's Mr. Fiorentino said that with 55 dispatchers on each eight-hour shift, the company ++ has "adequate human resources" to monitor the system. The dispatchers are also familiar with the area they cover.
"The dispatcher monitors every train movement and every signal position on the territory," he said. In the state-of-the-art computer center, opened in 1988, dispatchers perform similarly to air traffic controllers.
If the switch in the Friday accident had been frozen shut or disabled, that would have shown up on the dispatcher's screen in Jacksonsville, Mr. Fiorentino said. The dispatcher would then have been able to use a radio or signals to try to prevent the collision, he said.
Even so, Mr. Mann said, Amtrak should have been able to stop its train.
Amtrak officials said they could not comment because of the investigation.
The 'push-pull' method
Mr. Mann said the system whereby a locomotive pushes the train -- passenger cars in front with the engine at the rear -- is dangerous because passengers are left unprotected by the heavy engine.
In this method, known as "push-pull," a commuter train that pulls into a station locomotive-first doesn't have to turn around to retrace its route. The train simply leaves the station with the engine in the last position, pushing the other cars forward.
Not having to turn around saves time and money.
"There is absolutely no reason safety-wise why they should allow a train to be pushed, other than simplicity of operations," said Mr. Mann.
A spokesman for the Railroad Administration defended the "push-pull" method, saying it has a good safety record among commuter rail lines from Southern California to Boston.
Luis del Rio, a spokesman for the federal Railroad Administration, said that the administration is actively working on several safety issues, including standards for passenger rail cars. In 1994, Congress mandated that such standards be developed, and Mr. del Rio said the railroad agency expects to make public a report within the next few weeks.
The entire track from Brunswick to Washington has been upgraded in the last several years, work that included improving the signals.
CSX's Mr. Fiorentino said the changes enable the tracks to carry more trains. In 1993, MARC operated 28 daily trains, compared with 40 now.
Major Amtrak crashes
Some recent major Amtrak crashes and derailments in which people have been killed:
Oct. 9, 1995: One person was killed and 78 injured in the sabotage of an Amtrak train in the Arizona desert. The train was en route to Los Angeles from Florida. Investigators found a typewritten note near the derailment suggesting federal agents intentionally spread the fire that killed about 80 people at the Waco Branch Davidian complex in Texas two years earlier.
June 8, 1995: Seven members of a farming family were killed when an Amtrak train slammed into their pickup truck at a rural crossing equipped with stop signs in eastern Oregon, near Nyssa.
May 16, 1994: An engineer was killed and more than 350 people were injured when an Amtrak train bound for Florida from New York hit a truck trailer that came loose on a passing freight train. The accident occurred near Smithfield, N.C.
Sept. 22, 1993: Forty-seven people were killed near Mobile, Ala., when a tugboat smashed into a river bridge and caused it to collapse as an Amtrak train was passing over it. It was the worst accident in Amtrak's history.
March 17, 1993: Six people were killed and a dozen injured when an Amtrak train rammed a Hess Oil gasoline truck at a railway crossing near Fort Lauderdale, Fla.
Jan. 4, 1987: Sixteen people were killed and 170 were injured when an Amtrak train collided with the rear of three Conrail locomotives and derailed in Chase, Md.