Amtrak revived the defunct Washington-to-Florida auto-ferry service yesterday when its first Auto Train express left the Lorton, Va., terminal for the overnight trip to Sanford, Fla.

Amid great celebration, officials of the federally subsidized passenger railroad confidently predicted that their auto-ferry service would succeed where the previous, privately owned endeavor failed.

The former service, run by Auto-Train Corp., went bankrupt in 1981, after the company had accumulated at least $20 million in debt.

However, Amtrak officials are convinced they can make a go of their service, saying their costs will be lower since they already have the necessary organization in place to run the unique rail service.

"We fully expect that the Auto Train will be profitable, said Amtrak Group Vice President William Norman before the train left. Noting that yesterday's train was sold out, Norman added, "the interest and enthusiasm we have seen thus far is encouraging."

Trains will leave from Lorton at 4:30 three afternoons a week--Sunday, Wednesday and Friday--and arrive in Sanford at 9:30 a.m. the next day. Northbound, trains will leave Sanford Tuesdays, Thursdays and Saturdays, also at 4:30 p.m. Daily service is scheduled to begin March 1.

Passengers are required to travel with their cars, and the fares for a one-way trip are about 30 percent more than those charged when the last Auto-Train ran. They are: $200 for the car, $130 for each adult and $98 for each child between two and 11. There are extra charges for passengers wanting sleeping accommodations.

Reservations are required for the service, which can carry a maximum of 264 cars and 476 passengers.

To launch Auto Train, Amtrak paid the bankrupt company about $2 million for the use of the name and for the 19.8-acre terminal at Sanford. It is leasing the Lorton facilities, the same ones used by Auto-Train. Total start-up costs are expected to be $4.4 million.