Evidence that Amtrak workers failed to connect properly th air brakes of four locomotives that crashed into a Conrail freight train near Chase on Friday turned the investigation yesterday to Amtrak's rail yard near Washington's Union Station.

Alan Pollock, of the National Transportation Safety Board, said investigators will begin questioning this week the workers at Washington's Ivy City Yards who prepared the train -- a diesel locomotive hauling three idle electric engines -- for the trip to Philadelphia for maintenance.

The massive locomotives, their brakes screeching, slid past at least one red signal and slammed into the 64th car of a 125-car Conrail coal train about 3:09 a.m. Friday, investigators said. The crash occurred as the Conrail train was crossing the locomotives' path at a switch just south of the Gunpowder River.

That switch is about 100 yards from the site of an eerily similar accident -- the 1987 wreck of the Amtrak Colonial, which rear-ended a Conrail train that blundered onto its path. Sixteen people were killed and at least 170 injured.

The Amtrak locomotives were not pulling any passenger cars Friday. The only injuries were to the Amtrak engineer, Ray Francis Hunsberger, 38, and the conductor, Ronald Edward Hairston, 48, both of Pennsylvania.

They were listed in fair and stable condition at the Maryland Shock Trauma Center in Baltimore yesterday afternoon.

Amtrak service between Wilmington and Baltimore, halted for more than 12 hours Friday, resumed its regular schedule after 11 a.m. yesterday when two of four closed tracks were reopened, said a spokesman for the passenger rail agency said. The other two tracks, used mostly by freight trains, were expected to reopen late last night.

James E. Burnett Jr., a member of the five-member safety board who is leading the investigation, told reporters at a news conference Friday night that the air brakes on the three "dead in tow" electric locomotives had been improperly linked to the air brakes of the operating diesel.

Because none of the three electric locomotives was running, none was pumping air into the reserve tank for its air brake system, he said. A lever on each of the electric locomotives, he said, should have been turned from horizontal to vertical to permit the diesel engine to replenish the electric locomotives' reserve air tanks.

But investigators at the crash site found all three levers set in the horizontal position -- cutting off air to the reserve tank.

The Amtrak locomotives stopped at least once after leaving Ivy City at 12:18 a.m. Friday, investigators noted. But Mr. Pollock said the train's brakes would have been able to operate properly a few times by using up air left in the reserve tanks.

Once that air was exhausted, though, the only usable brakes would have been the diesel's. At high speed, investigators said, the diesel would have had difficulty stopping the massed dead weight of the three electric locomotives behind it.

Mr. Pollock said the investigation in the Washington rail yard would focus on finding out who prepared the locomotives for departure, what procedures should have been followed and what kind of checks are in place to ensure they are carried out.

NTSB also wants to know, he said, whether the Amtrak train's crew, as required by regulations, either checked personally that the brakes on their locomotives were properly hooked up, or was informed -- either verbally by a supervisor or in written form by mechanics -- that brakes preparation was done correctly.

Investigators have just begun analyzing data recordings from the cabs of the Amtrak and Conrail trains, Mr. Pollock said. The analysis will provide evidence of the speed of the trains and actions taken by the crew prior to the crash, he said.

Results of drug and alcohol tests on the Amtrak and Conrail crews are not expected for several days, he said. A formal finding of the cause of the accident may not be issued until early 1992.

R. Clifford Black IV, an Amtrak spokesman, said yesterday that his agency had no response to the NTSB's evidence that the brakes were improperly connected. "The preliminary statements made by the NTSB are certainly within the realm of possibility," he added.

If it turns out Amtrak workers were at fault, then there are several steps the rail passenger agency could take, Mr. Black said. "You re-assess your procedures to make sure that they are ironclad," he said. "Perhaps you add a layer of checks and review your procedures."

Roger A. Horn, the Johns Hopkins University mathematics professor whose 16-year-old daughter, Ceres, died in the 1987 wreck of the Colonial, said yesterday that there were "striking similarities" between Friday's wreck and the crash that killed his daughter.

"In both cases, the trains were allowed to leave and begin their trips with their important safety systems inoperative," said Dr. Horn, who helped found the transportation safety lobbying group Safe Travel America.

In 1987, a whistle to warn the crew of the Conrail train if they passed a red signal was found wrapped in tape and muffled. A light on the train's --board to warn the crew to slow down was not working.

"Someone was supposed to set up those hoses and set those levers" on the Amtrak locomotives Friday, Dr. Horn said. "I think Amtrak needs to examine its procedures for its pre-departure check-out of its trains."

Safe Travel has fought to bar freight trains from operating on the same tracks as high-speed passenger trains. Dr. Horn said the Northeast corridor between New York and Washington is the only place in the country where this occurs.

"I think that's a desired goal, but I don't think it's attainable at least near term," said Amtrak's Mr. Black. "There are approximately 150 freight customers who are served by the Northeast corridor," many of them major employers, and they have no alternative rail service.

Mr. Burnett said Friday that drug and alcohol tests were performed at 5 a.m. on the Amtrak crew, but Conrail delayed ordering similar tests until nearly 11:30 a.m.

Craig MacQueen, a Conrail spokesman, said his agency's officials did not think toxicologic tests were necessary because the freight crew showed no signs of impairment, and it was obvious they were not at fault.

The Conrail crew was tested only after estimates of the damage reached $790,000. Federal law requires testing when damage exceeds $500,000.