Amtrak announced a present to the nation's legion or rail buffs yesterday - a special transcontinental steam passenger excursion.

The event, which Amtrak said could be the last such steam run in American history, has been made possible by the necessity of returning the American Freedom Train's locomotive to its home base in Portland, Ore.

Once a mainstay on the Southern Pacific's Portland-Los Angeles route, the steam engine led the Freedom Train across the country during last year's Bicentennial, displaying Americana and historic documents.

Known as SP 4449, the engine weight 425 tons including a coal tender. It began service in 1941.

Amtrak said it will run the trip in two phases, starting at 8 a.m. on Jan. 14 in Miami, when the engine will pull an 18-car train over the Seaboard Coast Line to West Palm Beach, Winter Haven, Orlando and Sanford, Fla., for an overnight stop.

On the following two days, the train will move northwest to Jacksonville, Valdosta, Thomasville, Montgomery and Birmingham, partially over the Louisville & Nashville. The engine will remain at Southern Railway's locomotive shop in Birmingham until April and then will continue to Portland.

Amtrak said the routing for the Birmingham-Portland trip till is being planned. Passengers traveling for the first three days, covering 900 miles, will pay a special fare of $80 ($70 for children). For shorter distances, the fare will be $30 a day $24 for children).

Amtrak said tickets may be purchased at stations along the route or by sending checks to Armtrak, 1721 K St. NW, Washington, D.C. 20006.