The cost of traveling on Amtrak trains will be much cheaper this fall under several new fares announced yesterday by the railroad. The new fares are designed to be competitive with recently reduced airline rates.

Effective Sept. 6 will be a new excursion plan that will slash round-trip fares by as much as 40 percent, a reduction in thecost of the U.S.A. Rail Pass and a less expensive family plan.

Under the new excursion plan, the return portion of a roundtrip ticket will be priced between $5 and $20, depending on the cost of the one-way fare. The new round-trip excursion rates will apply only to trips of at least 250 miles on one train and will not apply in the Northeast corridor.

Prices of the U.S.A. Rail Pass, which allows nearly unlimited travel, will drop substantially this fall. Prices will drop from $250 to $169 for a 14-day pass from $315 to $219 for a 21-days pass and from $385 to $259 for a 30-day pass. Rail Passes are good for coach travel on all Amtrak and Southern Railway trains except Amtrak's Metroliner.

Families traveling together on Amtrak will also find fares cheaper this fall. The family plan discount for spouse and children aged 12 to 21 will increase to 50 percent from the present 25 percent. The discount for children aged two to 11 will increase to 37.5 percent from 25 percent. The discounts apply to a spouse and children traveling with a head of household who pays full fare.

Amtrak's Vice President of Marketing Al Michaud said that although Amtrak's major target is the auto traveler, "the new structure is also designed to be competitive with sharply reduced airline fares."

Amtrak decided to extend the new excursion fares with fixed return-trip costs system-wide after the concept proved successful on several individual trains, such as the New York-Montreal Adirondack train, on which ridership increased by 100 percent when a similar excursion plan was instituted in 1977.