PLANNING a trip to Florida? How are you going to go? By car or by air? For most travelers who live in the Northeast Corridor, those are the only alternatives usually considered.

Unless you're a masochist and want to go straight through, driving to Florida is a two-day trip. And since you've got to go through the same routine in the other direction on the way home, driving is hardly feasible if all you've got is a week or 10 days (though many people do it anyway for less than a two-week vacation to save money).

Flying is definitely quick, usually less than three hours from anywhere in the Northeast to any city in Florida, but it can be expensive -- especially for a family. And once you get there, you face the added cost of renting a car.

Well, there is a third alternative that can be both faster and easier than driving and, at the same time, less expensive than the combined cost of flying and renting a car.

It is called the Auto-Train and it is just what its name implies. It is a train that transports both you and your car over the roughly 900 miles between Lorton, Va., and Sanford, Fla. (The Lorton terminal is located six miles south of Washington's Capital Beltway, just off Interstate 95. Sanford is roughly halfway between Orlando and Daytona Beach.)

Now in its ninth year of operation, this privately owned corporation operates daily trains, both northbound and southbound, between its own terminals. Either way, trains are scheduled to depart at 4 p.m. and arrive at 9 a.m. the next day.

That figures out to 17 hours of traveling and that's about what it is, regardless of when the Auto-Train actually gets under way. On a recent excursion, it was 30 minutes late laving Lorton and 35 minutes late arriving in Sanford. (There have been, however, reports of delays of 90 minutes and more in departure and arrival times.)

But there is more to it than that. Auto-Train passengers are required to arrive not later than one hour before scheduled departure, and those who elect to board as late as possible will have the worst choice of seats and will find themselves probably dining either far sooner or far later than they would like.

We arrived at the Lorton terminal at 1 p.m., three hours before departure, and, after clearing up what could have been a disastrous ticketing error, found things a beehive of activity. Even at that early hour, the 6 p.m. dinner seating was filled. We settled for 7:30, while later arrivals would get their choice of 4:30 or 9 o'clock.

At the end of the trip, depending on the luck of the draw, you might spend up to two hours waiting for your car to be unloaded from the car carrier. All together, it can be a 22-hour experience.

In itself, the Auto-Train can hardly be considered inexpensive. Our family -- two adults and three children -- spent $479 for a one-way trip from Lorton to Sanford. Of that, $115 was optional: We wanted the privacy and convenience of a four-bed stateroom (the two youngest children shared one bed). c

The price breakdown included $119 for the automobile, $70 for each adult and $35 for a child 17 and under. For that, the accommodations include reclining lounge seats with extendable leg and foot rests, along with a pillow and blanket.

In addition to the stateroom, a two-bed bedroom is available for $85 and a three-bed drawing room is available for $100. Those two rates are unchanged.

(Other rates have risen slightly, and a new policy sets higher fares for northbound trips in certain months due to less demand for seats in that direction. Currently, from June 1 through Sept. 30, the charges (both northbound and southbound) are: $139 for auto, $75 for each adult and $38 for each child after the first two, which travel free with an adult. There is a 30-percent discount on return trip made within 31 days during that four-month period.

(Auto-Train recently welcomed its "2-millionth passengers," a couple from Pennsylvania, and President Eugene K. Garfield noted in ceremonies at Sanford that "every person who travels by Auto-Train . . . eliminates 900 miles of highway driving and saves approximately 60 gallons of gasoline.")

At the base fare, our trip would have cost $364, which is roughly comparable to current airline discount fares for a family of five. But we also had our car, eliminating the cost of a rental, which now runs about $120 a week for a mid-size sedan.

By Comparison, our drive home two weeks later cost about $150, including $35 for dinner and $35 for a motel room in North Carolina, $65 for gasoline, $12 for breakfast the second day and about $3 for tolls. This does not include the cost of breakfast and lunch on the first day or lunch on the second day, because they would not have been included in the Auto-Train fare and would have had to have been purchased anyway.

We made our reservations four months in advance by telephone (800-424-1111).

Except in times of gas shortages, regular accommodations are rarely sold out, although the Auto-Train people recommend at least a two-month advance reservation for bedrooms.

After making the telphone reservation, Auto-Train mails you a confirmation, requiring that a deposit of $50 for a one-way trip ($100 for round trip) be mailed within 10 days.

After getting out the personal items we would need for the trip, we turned the car over to an attendant, who examined it for damage (if there is any, it is marked on a form that you and he sign) and he put it in line to be loaded on the car carrier.

At about 2:30, we were told the train was ready for boarding. Our compartment (calling it a stateroom is taking too much license) was located in the second car behind the twin diesel locomotives. It consisted of two bedrooms with a center divider folded away and was about 8 by 10 feet. The lower berths were folded up into couch positions and there was a padded folding chair in the center -- enough room to seat five. There were two lavatories and two smaller closets.

A young man came by and introduced himself as our attendant for the trip. We asked for ice but he said it would be an hour or so before it was delivered. We got our ice shortly before the scheduled departure time, but for some reason or another it was another half hour before the train pulled out.

At first, we thought it was the flush of excitement, but as time wore on it became obvious that the heating system was working overtime. There were temperature controls, but no matter where we set them it was warm. As we were to find out later, people in the sleeper car behind us had the opposite problem -- no heat.

We watched the passing countryside for awhile but the children soon got edgy. Fortunately, my wife had thought to bring a deck of cards and we all played rummy. Shortly after 6, we convinced the kids to take a little nap, and after having the attendant turn down all four beds, we headed for the lounge car and a cocktail before dinner.

After returning to the compartment and freshening up, we set out for the dining car at 7:15. We were seated and a waiter was quickly in attendance. The choice of the entrees included chicken, fish, beef stew and salisbury steak. Nothing elaborate, but since dinner and the continental breakfast the next morning are included in the fare, it was adequate.

Dinner also included a salad of crisp lettuce, rice, mixed vegetables, rolls, chocolate cake and a choice of milk, tea or coffee. Beer and California wine were available at $1 per serving. Considering the Auto-Train is catered by one of the same companies that serves airlines, it was a surprise to find that the food was quite good.

A movie is shown (on wall-mounted TV screens) in each of the two dining cars, but it doesn't start until 10:30, following the last dinner seating and clean-up. We decided to pass up the late show, spending the evening playing cards and other games with the children before bedding down before 11.

Since our 5-year-old has a built-in alarm clock that wakes him when the first ray of sunlight penetrates the eastern sky, we all were up before, somewhere near the Georgia-Florida border. Later, as we prepared for our 7:30 breakfast seating, the train moved through Jacksonville. Breakfast consisted of orange juice, Danish pastry, croissant with butter and choice of coffee or milk.

At 9:35, the train pulled into the Sanford terminal and, shortly before 11, our car came down the ramp from the carrier and we set out on the 45-mile drive to Walt Disney World.

Our family had mixed feelings about the Auto-Train. The kids thought it was an exciting adventure most of the time, although the last three hours seemed to drag interminably for all of us. Both the food and service turned out better than we expected. And we all agreed on one point. Like the Grand Canyon, Niagara Falls, the Indianapolis 500 and the Kentucky Derby, the Auto-Train was something we wanted to experience. But we doubt we'd do it again.