The likelihood of two Port Authority Light Rail Transit vehicles crashing into each other is slight, officials said yesterday, but to say "it can't happen here" would be an overstatement.

In fact, it did -- on July 25, 1995, when one train rear-ended another near the Dawn stop on the Beechview line, injuring 139 passengers.

But there are significant differences in how the trains operate here and on Washington, D.C.'s, Metro system, where one train rammed another Monday, killing nine people.

Metro rail cars typically are run by onboard computers that control speed and braking, with the operators required only to close the doors after stops, The Washington Post reported yesterday.

LRT vehicles here are controlled by the operators, who must obey computerized signals on much of the system that are designed to keep vehicles a safe distance apart.

If a train continues through a red light, an electromagnetic device in the track sends a signal that automatically brakes the vehicle to a stop, said Winston Simmonds, Port Authority's operations manager.

If there is an equipment failure, the signals are designed to default to red. In a power outage, operators are trained to treat an inoperable signal as being red, and if they try to pass, the car will be automatically halted.

But portions of the LRT system don't have signals -- including the stretch from South Hills Junction to the South Busway Bridge, the section of Broadway Avenue that the LRT system shares with automobile traffic and the Library line south of Washington Junction.

In those zones, the operators are responsible for staying a safe distance from the train ahead and should be able to stop in half of that distance or less, Mr. Simmonds said.

Pittsburgh's rail vehicles operate at much slower speeds than Washington's, with a maximum of 30 mph and an average of 12.6 mph, while Metro trains have a 59 mph maximum and 33 mph average.

The positions of all vehicles are monitored at the authority's Operations Control Center at South Hills Village.

The vehicles can be seen as numbered white blips on a 40-foot long electronic display board that also shows the status of each signaled section of track.

Employees called "movement directors" sit at consoles that vaguely resemble those in NASA's control rooms. They are able to manually control signals and traffic if there is maintenance work or in other unusual circumstances, said Todd McElravy, assistant manager of rail operations.

"We have control of everything out here," he said.

Soon to debut on the newer Overbrook line is an automated system that delivers information to operators on the maximum safe operating speed, based on their position and those of other vehicles. If an operator exceeds that speed, the system will automatically bring the vehicle to a stop, Mr. McElravy said.

The worst crash in the 24-year history of the LRT system occurred at the entrance to a construction zone where the automatic stop feature at a signal was intentionally disabled to allow smoother traffic movement.

Because of the construction, trains in both directions operated on a single track, and outbound vehicles were queued up near Dawn to wait for inbound trains to pass.

The operator of a 47S South Hills Village car was supposed to stop at the red signal just before the construction zone, then proceed slowly to the queue.

He told investigators that he blacked out and continued through the signal without stopping. With the automatic stop system switched off, the train rolled past the red light at 15 mph and rear-ended a stopped 42L Library vehicle.

The National Transportation Safety Board concluded that the authority shouldn't have disabled the automatic stopping system and also faulted the driver.

Port Authority officials would not speculate on the cause of Monday's crash in Washington but said they would try to learn from it.

"When you have a tragic accident like this in the industry ... you take a retrospective look at your own operation," Mr. Simmonds said. "We'll be waiting for the investigation to come out and look at the recommendations and see how we can apply them to improve what we're doing here."