Federal investigators said yesterday that apparent engine trouble on the Chicago-bound Broadway Limited triggered a series of events that culminated Friday in the collision of two other trains in which 68 passengers were injured.

The accident occured at 6:35 p.m. less than a mile from the Seabrook station in Prince George's County when a four-car Conrail commuter train struck the rear of a 14-car Amtrak train, the Montrealer.

William Pugh, the chief investigator of the accident for the National Transportation Safety Board, said yesterday that the stoppage of the Broadway Limited south of Bowie set off automatic signals that several miles behind it to halt.

When the problem on the Broadway Limited was fixed, Pugh said, the two following trains were allowed to proceed, but the Montrealer then developed engine trouble.

"When the engineer on the Montrealer started to pick up speed he had problems, with his traction motors (which turn the wheels) so he slowed down." Pugh said.

Moments after the Montrealer left the Beltway station the Conrail train arrived there and found "stop" signal. Pugh said engineer Hugh V. Hoffmaster told him that 90 seconds later he received a "stop and proceed," signal that allows the train to continue, but not faster than 15 miles per hour.

"By now the Montrealer was approaching the Seabrook crossing." Pugh said. "It was almost stopped, probably going less than five miles an hour since the engineer was trying to stop short of the crossing."

The Contrail came around a bend seconds later, engineer Hoffmaster saw the stricken Montrealer and tried to apply his emergency breaks.

A split second later the Contrail slammed into the heavy mail car at the rear of the Montrealer and rebounded several feet on impact.

"Train three (the Conrail) couldn't have been going more than 15 miles an hour at impact," Pugh said. "If it had, the damage to the trains and the tracks would have been much worse than it was."

Following the accident, 68 passengers were taken for treatment to Doctor's Hospital of Prince George's General Hospital. Only Mary Edwards, 58, of Pennsylvania, was admitted and she was being treated yesterday for high blood pressure and minor back injuries.

Pugh said that he will begin an extensive investigation Monday of the incident. This will include inspection of all three trains, especially the engine of the Montrealer and the brake system of the Council train.

Yesterday the Montrealer was taken to Wilmington, Del., for examination while the Conrail train was taken to Union Station.

"We'll be interviewing the crews from all three trains and the inspectors who examined the trains before they left. He'll also check the signaling systems on the tracks and listen to a tape of all the communications that went on prior to the accident," Pugh said.

He said he expected the investigation to take about 60 days to complete.

Amtrak officials said yesterday that full service on the three tracks between Washington and Baltimore had been restored by mid-morning.

Crews worked througout the night repairing damaged portions of the track.After the crash several hundred spectators gathered on the hills overlooking the scene. As midnight approached almost 100 people still were watching crews work.

"Did I hear the crash? How could you not hear it," said Prince George's County fireman John Hudges. "I was in my house three blocks away and it sounded like an explosion right next to me."