Norfolk Southern Corp. and CSX Corp. yesterday limped to the end of a third week of congestion and service breakdowns growing out of the complicated merger with Conrail, but there were signs the problems may be stabilizing.

Some customers of both Norfolk Southern and CSX reported improved service, while others reported continued deterioration, including the railroads' single largest customer, United Parcel Service of America Inc.

Customer reports come in varying shades of alarm or ease. UPS, which not only is the railroads' largest customer but the most demanding, said both railroads were running hot UPS trains three to 13 hours late, threatening to shred UPS's customer guarantees.

"For us, that is just unacceptable," said UPS spokesman Norman Black, who said the company has crammed as many of its brown trucks as possible back onto the highway. He estimated that perhaps half the UPS trucks that were on rail cars before the June 1 merger -- mostly on CSX -- are back on the road.

Black said service is not in a meltdown. "We do view this as a temporary problem. We are upset it happened. We did not expect it to happen this way."

However, Ford Motor Co., one of Norfolk Southern's largest customers, said that while service is still a problem and some plant overtime work has been cut back for lack of supplies, there have been no plant shutdowns. "We don't consider any of this important at this point," Ford spokesman Ron Iori said.

A snapshot of system statistics, while inconclusive in the short run, indicates that things are not getting worse. A key measure of congestion, the number of cars on each railroad that have not been delivered to customers, stabilized over the past few days. But some key yards remain congested on both railroads, and freight train crews are sometimes unable to complete trips before they reach the maximum 12 hours on duty under federal law.

In an effort to assure that exhausted crews work through the weekend to whittle down a backlog of traffic, both railroads have called on their unions for help.

Norfolk Southern, which traditionally has had a less friendly relationship with national rail unions than CSX, has asked for help and has offered a bonus of up to double pay for any union member who works through the weekend to help clear the traffic backlog.

"We're seeing the beginnings of change over there [at Norfolk Southern]," said Charles Little, president of the United Transportation Union, who said the union is doing everything possible to help the railroads.

Both railroads, which split the eastern railroad Conrail between them on June 1, have made headway in fixing dumbfounding computer problems that have sent thousands of freight cars astray. This included an odd quirk on Norfolk Southern called the "ping-pong effect," in which a computer declares a loaded car suddenly empty and orders the car returned to its original terminal.

On CSX, computer systems that had seemed to be operating properly last week suddenly spent a couple of days selectively misrouting hundreds of cars.

Norfolk Southern still appears to be in worse shape than CSX, but Richmond-based CSX also suffers congestion problems that have resulted in deteriorated service.

Ronald Conway, CSX executive vice president for operations, said customers on former Conrail lines are not getting the service they received under Conrail. "I can't say when we will get back up to Conrail service," he said. However, "we're going back in the right direction."

The railroads have invested in hurried construction projects to relieve seriously congested yards. And they have shifted large volumes of freight to alternate routes on smaller railroads, effectively reactivating former main lines that were declared surplus.

"There's a lot of track still out there," said James W. McClellan, Norfolk Southern senior vice president. McClellan said formerly excess infrastructure in the East may be one of several important reasons that it will be spared the fate of service meltdown that hit the West after the Union Pacific's 1996 merger with Southern Pacific.

Alan W. Maples, president of the Everett Railroad Co. of Duncansville, Pa., one of many short-line railroads that have suffered serious service disruptions, said Norfolk Southern computer systems can at least now tell him the location of almost all of his incoming cars. "I'm feeling better today," Maples said. "I hope I'll feel much better next week."

CAPTION: A CSX train travels over the Kanawha River in West Virginia. CSX computer systems misrouted hundreds of cars this week.