THE BLAST from the locomotive's whistle is insistent, piercing the quiet of a fall morning. Startled youngsters, waiting on the station platform, gape in awe. Their parents clasp hands to their ears. Nostalgia buffs, who have traveled hours to hear this sound, coo in delight.

With a wave, the conducter hurries aboard the stragglers. In a few moments, the massive engine lets loose one more hoot, shudders muscularly in anticipation and then in a clamor of gears and pistons lurches forward on a rollicking ramble into a bygone era, passengers grinning from every window.

Within a half-day's drive of Washington, at least eight shortline steam railroads provide regularly scheduled roundtrips into the countryside until the end of October. It's a pleasant, and inexpensive, way to enjoy the colorful autumn foliage on an overnight or weekend outing.

Among the choices for an old-fashioned train ride: four railroads in Pennsylvania and one each in Virginia, West Virginia, Delaware and New Jersey. They pass through some of the region's most scenic and historic areas, climbing mountain passes or winding along stream valleys.

A few trains run so frequently you can tote a picnic lunch, eat it at the end of the line and catch a later train back to your car. At the turn of the century, steam trains like these "carried passengers out of the cities to picnic groves, amusement parks and seaside resorts to escape the summer heat," say officials of the Wilmington and Western Steam Railroad of Greenbank, Del., which runs from outside Wilmington to just such a picnic grove.

These mini-railroads are as varied in their origins as in their itineraries. Most operate over track abandoned years ago by the big railways when the national rail system went into decline. Some are wholly tourist trains, manned summer and fall by volunteer rail fans; others are still-functioning branch freight lines, hauling heavy goods during the week and sightseers on the weekend.

The engines and the coaches are vestiges of decades past, in the days before air conditioning when you could raise a window to catch the breeze, or maybe a cinder in the eye. They stop now at quaint old stations, where gift shops have replaced the waiting rooms. Most offer a commentary, or conductors will answer questions. At clickety-clack speed, there's plenty of time to spot a deer or at least exchange winks with a lazy cow gawking over the fence.

The fall is the busiest season for many of the rail lines because of the changing foliage. "Autumn is just beautiful here," says Nancy Rowe of the East Broad Top Railroad in Rockhill Furnace, Pa., which also makes its turnaround at a country picnic grove. Despite the crowds, the trains, which carry up to several hundred passengers, do not reguire reservations for individuals and families.

The particulars:

* Cass Scenic Railroad, Cass, W. Va.: This restored logging train climbs 11 miles slowly through high spruce woodlands to Bald Knob, at 4,842 feet the second-highest point in the state, where the view on a clear day extends for miles in any direction. On this ride, the powerful Shay engine pushes the coaches up the steep grade, and the caboose actually leads the procession.

The Cass is owned by the state park system, acquired after the line, built in 1902, ceased hauling logs. In its day, say park officials, it moved "more than 2 billion board feet of timber from the cloud-covered ridges." The engines are the ones once used at Cass; the coaches have been converted from logging flatcars.

Fall Schedule: Weekends only through Oct. 30, except daily Oct. 6-9 and 13-16. Departure to Bald Knob at 12 noon for a 4 1/2-hour roundtrip; two-hour trips to Whittaker Station depart at 11 a.m., 1 p.m. and 3 p.m. (except on Thursday and Friday, when there is no 11 a.m. train). Fares: Roundtrip to Bald Knob, adults, $8.50; children (5-11), $3.50; to Whittaker Station, adults, $5.50; children $2.50; under 5 free.

For information: (304) 456-4300.

* Gettysburg Railroad, Gettysburg, Pa.: Originating from just one block north of the Gettysburg center square, the Gettysburg Railroad initially skirts the historic Civil War battlefield and then heads into the rolling Pennsylvania apple country. At times, a whiff of ripening fruit can overpower the acrid effusions of the coal-powered locomotive.

During the week, the line carries freight east from western Maryland. On weekends, the passenger run is a 16-mile roundtrip to Biglerville. And on Sept. 24 and Oct. 15, 16 and 22, there's a 50-mile roundtrip to Mt. Holly Springs (for which reservations are necessary).

Fall Schedule: Weekends through Oct. 30 at 1 p.m. and 3 p.m. for a 1-hour, 15-minute roundtrip. The special Mt. Holly Springs trips depart at 10 a.m., returning at 3 p.m. Fares: Roundtrip to Biglerville, adults, $3; children under 12 (occupying a seat), $1.75; to Mt. Holly Springs, adults, $8; children, $5.

For information: (717) 334-6932.

* Allegheny Central Scenic Railroad, Covington, Va.: A family-run line, the Allegheny Central was acquired by a former C&O Railroad machinist when the C&O sold the track. The railroad traces the meandering Jackson River from Intervale Station near Covington to the village of Jenkins Ford, a two-hour roundtrip of 24 miles. The route takes passengers under high cliffs and across pastoral valleys, always with the Blue Ridge Mountains as a backdrop.

Fall Schedule: In September, Tuesday and Thursday at 1 p.m.; Saturday at 10:30 a.m. and 1 p.m. and Sunday at 1 p.m. and 3:30 p.m. In October, Monday through Friday at 1 p.m.; Saturday at 10:30 a.m. and 1 p.m., Sunday at 1 p.m. and 3:30 p.m. Fares: Adults, $6; seniors (65 and older), $5; children (4-11), $3; under 4, free.

For information: (703) 962-2253.

* Strasburg Rail Road, Strasburg, Pa.: The Strasburg Rail Road's nine-mile, 45-minute roundtrip passes through the famous Amish farm country, originating at the East Strasburg station, 10 miles southeast of Lancaster, and running to the community of Paradise. The train makes a whistle stop at Groff's Grove, where returning passengers can stop over for a picnic and catch a later train. For rail fans, the Railroad Museum of Pennsylvania is right across the street from the Strasburg station.

Fall Schedule: Daily through Oct. 30; Monday through Friday, hourly from noon to 3 p.m. (last train); Saturday, hourly from 11 a.m. to 4 p.m.; Sunday, hourly from noon to 4 p.m. Fares: Adults, $3.50; children (3-11), $1.75; under 3, free.

For information: (717) 687-7522.

* East Broad Top Railroad, Rockhill Furnace, Pa.: The East Broad Top, a narrow-gauge line completed in 1873, hauled coal for almost 100 years from the Broad Top mines to the town of Mt. Union, where it was loaded onto standard-gauge Pennsylvania Railroad cars. The last three-foot guage line in the East still operating in its original location, it was rescued by a rail buff and is now a tourist line carrying 35,000 passengers a year.

Visitors can tour the original eight-stall roundhouse and watch the train's engine be prepared for its daily run when it pulls onto the turntable.

The 10-mile, 50-minute ride from Orbisonia Station along Aughwick Creek to Colgate Grove crosses terrain alternating between farmland and wooded mountains. Passengers can picnic at the grove, returning on a later train.

Fall Schedule: Weekends only through Oct. 30; hourly from 11 a.m. to 4 p.m. Fares: Adults, $4.50; children (5-12), $2.25; under 5, free.

For information: (814) 447-3011.

* Wanamaker, Kempton and Southern, Kempton, Pa.: Once a branch line of the Reading Company, where it hauled potatoes, farm equipment and passengers, the W.K.&S. is a tourist railroad operated by volunteers who rescued it from liquidation. It is also known as the Hawk Mountain Line because it is near the Hawk Mountain Bird Sanctuary.

Out of Kempton on the 40-minute, 6 1/2-mile roundtrip to Wanamaker, the train passes by a log cabin built in 1734 and then heads into the woods along the banks of Ontelaunee Creek. At Fuhrman's Grove, you can picnic by the creek in the shade of the trees.

Fall Schedule: Sundays only through Oct. 30; hourly from 1 to 5 p.m. Fares: Adults, $2.50; children (2-11), $1.25; under 2, free.

For information: (215) 756-6469.

* Wilmington and Western Steam Railroad, Greenbank, Del.: The Wilmington and Western, another volunteer line, crosses and recrosses Red Clay Creek several times on a series of wooden trestles. It is a nine-mile, hour-long roundtrip from Greenbank (near Wilmington) to Mount Cuba, a riverside picnic grove reached only by train.

Meandering through the hills and farmland of the Red Clay Valley, the train passes two covered bridges and, say train officials, the "remnants of the early water-powered industries which lined Red Clay Creek since Revolutionary times."

Fall Schedule: Sundays only through Oct. 30 at 12:30, 2 and 3:30 p.m. Special 20-mile fall foliage excursions to Hockessin, Del., are scheduled for Oct. 15 and 22, departing at 1 p.m. and returning at 4:30 p.m. Fares: Roundtrip to Mount Cuba, adults, $3; children (5-12), $2; under 5, free. To Hockessin, adults, $5; children, $4.

For information: (302) 998-1930 weekends.

* Black River and Western Railroad, Flemington, N.J.: Woodlands, streams and century-old homes brighten the view from the window of the BR&W, which hauls passengers Saturdays and Sundays and freight the rest of the week. On Saturday, the train makes a 12-mile roundtrip run to the station at Ringoes, site of a historic tavern; on Sunday, passengers can transfer to a self-propelled motorcar and continue another six miles (12 miles roundtrip) to the quaint town of Lambertville, just across the Delaware River from the antique shops of New Hope, Pa.

Fall Schedule: Weekends through Nov. 27 at 11:30 a.m. and 1, 2:30 and 4 p.m., transferring to Lambertville on Sunday only. Fares: To Ringoes, adults, $4; children (5-12), $2; children (3-4), $1; under 3, free. To Lambertville, adults, $7.50; children, $3.50 and $2.

For information: (201) 782-6622.

One thing about these coal-burning steam trains, older passengers are sure to remember, is that they can spew forth a steady drizzle of soot. At the end of the ride, count the number of smudged faces.