Now PATH engineers and electricians are struggling in the slime of pumped-out tunnels to restore power to electrical signal relays that flashed into green electrolytic mush when the highly-conductive salt water flooded the high-voltage circuits. They hope to restore service out of Hoboken by Monday.

Path officials say nearly 200,000 trips are taken through the tunnels each weekday.

Meanwhile, the system supervisors are conducting the first rounds of "lessons learned" briefings on what went wrong and what could have been done to avoid the shutdown. The discussion sometimes touches on how much worse it might have turned out. Preparations for Snow

"Sure, we've thought about what could have happened," said Michael A. Scott, general superintendent of PATH. "Fortunately we had some very reasoned minds working in the control towers who assessed the situation very quickly and moved very quickly."

Citing safety concerns, PATH officials would not allow a reporter to visit the tunnels.

In reconstructing the events of Dec. 11, Mr. Scott and others begin with a familiar complaint: early weather forecasts were wrong. Instead of cold and snow, the region got mild temperatures and blinding rain.

"We had people come in prepared for snow removal and to heat up the cars early, make sure the doors weren't iced up," Mr. Scott said.

In any case, it was not the rain that flooded the old railroad tunnels, some of which date back 100 years, but high tides that exceeded the limits of the National Oceanographic and Atmospheric Administration's gauges at the Battery and rolled like small rivers down the stair and elevator shafts at Hoboken and Exchange Place.

The Hoboken PATH station is at sea level at the edge of the Hudson. 10 Minutes to Trouble

The first sign of trouble came about 7 A.M., when the No. 3 track in the Hoboken station went under water. But, Mr. Priore said, the track lay in a low spot and had flooded before. The system's control center at Journal Square switched the Hoboken trains to tracks one and two, and sent out an alert for all trains to be on the lookout for flooding. It did not take long for them to find some, in the Meadowlands area outside Newark.