Today was the day the trains stood still.

At 4:30 p.m. all over the East, terminals stopped dispatching freight trains as computer specialists began preparations for the culmination of the most complex merger in railroading history. Rival rail giants Norfolk Southern Corp. and CSX Corp. officially divided up the operations of Conrail, paving the way for the first real competition in rail freight in the Northeast in more than 23 years.

Conrail was created by the federal government in April 1976 to take over several bankrupt eastern railroads, and its success was greater than anyone predicted. It began turning a profit in the early 1980s and after it was transferred to private ownership in 1987, it became a much sought-after takeover candidate.

Norfolk Southern and CSX battled mightily against each other for control of Conrail in 1997, eventually declaring a truce that gave NS 58 percent of the railroad's lines and CSX control of the remaining 42 percent. The companies paid a combined $10.2 billion for Conrail, hoping that by adding its lines they could offer shippers lower prices and more efficient methods of hauling freight.

Already, Norfolk Southern and CSX have started a multimillion-dollar rate war that has brought lower rates for shippers, and both railroads have added service guarantees that must be met. If more shippers choose to send more goods over the rails, however, both companies face challenges in managing the strains that extra business will place on the already heavily used rail lines in the Northeast and in the Washington area, which also carry commuters and Amtrak passenger trains.

"We're down to Railroading 101 now," said Norfolk Southern Chairman David R. Goode. He said it is up to both railroads to run their new systems efficiently and avoid the 1997 service nightmare that befell Union Pacific Railroad when it failed to integrate the computer systems of Southern Pacific.

Squads of Norfolk Southern and CSX officials and technicians gathered here today in the Blue Room, Conrail's operations center, to begin the changeover. "I feel like the captain on Star Trek, saying `make it so,' " said Goode, who flew from Norfolk and dropped by the work area in the nearly empty Conrail headquarters building to offer encouragement to the team.

Integrating the computerized information and control systems is one of the most complicated parts of the Conrail split-up. The three railroads had such different systems that it was necessary to scrap the Conrail system, which among other things was not Y2K-compliant, and bring all those functions separately into the CSX and NS systems.

But since the Conrail systems were needed for Conrail trains up to tonight's takeover, it was necessary in one day to tackle the huge task of shutting down 15 major Conrail systems, then shutting down CSX and Norfolk Southern computers to transfer the data, then bringing the systems back up and testing them.

Dispatching and signal systems were unaffected, so it was possible to continue running Amtrak and commuter train operations. But most freight trains could not continue operating for long because the inactive computers handled vital freight functions such as keeping track of locomotives and freight cars, calling crews to come to work, taking orders from customers and billing them.

At 3 p.m. today, technicians began backing up Conrail computer files, transferring data and shutting the systems down for the last time. All data were printed out, just in case there was trouble with the transfer.

At about 6 p.m., CSX computer systems were to go down for a planned nine-hour outage in preparation for the data transfer, synchronization of data and restart. Only a two-hour shutdown, planned for 9 p.m., was necessary for the Norfolk Southern computers.

Norfolk Southern systems were scheduled to come back up about midnight, and CSX computers at 4 a.m. Tuesday. By 7 a.m., if all goes well, crews are scheduled to be called back to work.

There were last-minute problems as Day One approached. Many Conrail crews decided to take days off at the last minute, stranding dozens of trains for lack of crews. Also, about 10 percent of the 600 Conrail managers who agreed to work for Norfolk Southern had changed their minds, including the entire management team at the giant Elkhart, Ind., freight yards.

There also was the threat of a Teamsters strike at midnight against car-hauling companies, which could cause a glut of new automobiles at railroad distribution centers, effectively blocking trains from unloading batches of new automobiles.

The Conrail split-up completes two decades of mergers among railroads, bringing the number of U.S. railroad companies down from dozens to four big systems -- Norfolk Southern and CSX in the East, and Union Pacific and Burlington Northern Santa Fe in the West. NS becomes a 21,600-mile railroad in 22 states, the District and Ontario, while CSX has 22,300 miles in 23 states, the District, Ontario and Quebec.