In 1906, a traveler on his way from Washington to Alexandria would have turned his horse and buggy south on the Alexandria Turnpike soon after crossing the Polomac River. As he drove through the marshy farmland the past the brick kilns where Crystal City is today, he might have wondered at the construction he saw between the turnpike and the river just south of the Fourt Mile Run.

What he would have seen was the Potomac Yards freight interchange, being built by the Richmond, Fredericksburg and Potomac (RF&P) Railroad to receive and reroute freight to destinations north and south of Washington.

Today, a commuter driving home along the George Washington Parkway might be equally puzzled by the construction near potomac yards. Not only is Metro working to extend its Blue Line from National Airport to Alexandria, but construction on the Four Mile Run Flood Control Project is in full swing.

Caught in the midst of these two projects and surrounded by development on all sides is the Potomac Yards.

The intense development along that stretch of the George Washington Parkway has been intertwined with the history of the Potomac Yards and of the railroading industry.

Records show that in 1916 there were 450 acres in the Yards. This acreage was added to until it reached a peak of 700 acres in the 1940s. During World War II activity in the Yards reflected the country's dependence on the railroads. At one point during the war, 9,000 cars a day - including troop trains and rerouted passenger trains - were moving through the yards, according to news reports. But in the last 20 years, as the pressures of taxation on its land increased and the automation of railroading took hold, RF&P was forced to divest itself of much of the land that once was the yards - now down to 526 acres - a move which in turn led to the development of Crystal City and other tracts along the parkway.

Today, the RF&P is an anomaly in railroading, a prosperous and growing business. The Yards have prospered with the railroad, observers say, mainly through astute land usage and improvements allowed by modern technology.

Stuart Shumate, president of RF&P, is guardedly optimistic about the future prosperity of the Yards. "What happens in the Northeast depends on things we won't be able to control. It depends a lot on government policies. Amtrak owns the Northeast rail corridor and successful freight service will depend on what priority the government puts on passenger transport over freight."

He also points out that the manufacturing centers in the United States are shifting from the Northeast to the Southwest, which means that what is manufactured in Texas will have to shipped back East - Probably by rail.

In addition, the RF&P still has one prize piece of real estate along the V.W. Parkway - 38 acres between the RF&P tracks and the airport - which it plans to develop in the future. Shumate will say little about the railroad's plans for the parcel other than it will "probably be leased" to a developer as was done with the Crystal City tract.

In the 1950s, the RF&P, faced with increasing real estate taxes, began divesting itself of land at the Yards. The tract where the 14th Street Twin Bridges Marriott Motel now stands was sold by the railroad. The potential development value of land in the Yards on which its taxes are based varies in relationship to the assessed value of the property surrounding it. This is as high as $16 to $18 a square foot at the northern end bordering on Crystal City and as low as $6 a square foot at the southern end around the Monroe Avenue Bridge. In 1975, taxes on land in Potomac Yards exceeded $1 million.

In the 1960s, the railroad gave a long-term lease to Charles E. Smith and Co. for the land between 15th and 21st streets in Arlington for development of a large part of Crystal City.

In a similar decision in 1971, RF&P leased 38 acres between the northbound classification tracks and the G.W. Parkway to Charles Fairchild and Company-Fairchild's plan to build a high rise commercial center there, however, has been delayed, in part, by court challenges from local citizens' groups.

Even if Fairchild's plans are not completed, the railroad will continue to search for alternate uses for its land, railroad spokesmen say.

When the Potomac Yards were first built, they were only one of three freight interchanges near Washington including the Hagerstown interchange, run by the Baltimore and Ohio Railroad and still in operation today, and the Norfolk interchange, which handled freight for the DelMarVa Peninsula. The building of the yards marked the first time that more than two or three railroads shared a freight classification facility. Five railroads shared the yards in those early years - the RF&P, the Pennsylvania Railroad, the Southern, the Baltimore and Ohio and the Chesapeake and Ohio.

As the landscape has changed around the Potomac Yards with RF&Ps ventures into real estate, so have the ways of railroading changed.

The yards now handle about 4,000 cars each day of the year. According to Shumate, the volume of cars passing through the yards has not changed muchin five years, but the average load per car is much greater. The use of "piggybacking" - a system by which large containerized freight trailers are loaded on flatcars for part of the journey - has been increasing by 20 percent a year.

Although the Yards handle fewer cars now than during World War II, the volume of goods has increased because of more efficient methods of handling freight, said Yards Superintendent John McGinley.

Six railroads share the yards facilities. Conrail has the largest volume of traffic, with 34 percent of the total; RF&P has 27 percent; Souther, 21 percent; Baltimore and Ohio, 12 percent; Chesapeake and Ohio, 5 percent, and Delaware and Hudson, 1 percent.

From the ground, the Yards appear incomprehensible, but from the control tower the maze of tracks begins to take shape.

The Yards are divided into two sections: northbound, with 18 receiving tracks and 50 classification tracks; and southbound, with 13 receiving tracks and 39 classification tracks.

Nearly all the receiving, classification and rerouting is now done by computers. While the classification clerk assigns each car to a track, cars are inspected for mechnical defects. Until the 1950s, a pit inspector performed those checks by sitting in a hole and peering up at the undercarriages of cars as they rolled above him.

As the cars approach the "hump," a slight incline that separates the receiving the classification tracks, an automatic route selection system is activated from a control panel in the hump office. As the car moves over the hump, its weight and speed are measured and fed into an electronic speed-control apparatus.

On the other side of the hump, the car's progress toward recoupling with other cars, primarily its speed, is controlled by automatic car retarders.

Kemp Rush, 87, who worked at the Yards for 50 years, remembers when it took many more brakemen and switchmen to complete the now-automated system. And he remembers the dangers.

"In those days," he said, "there were 18 men on each 12-hour shift at each hump. Several men got killed while I worked there, usually brakemen who fell from the tops of cars where the manual brakes were."

Today, shifts are eight hours long, and a hump crew consists of five workers. Two other innovations helped the Yards reduce its labor force, which was at a peak of 1,600 in World War II and is now 490. The introdduction of the diesel engine in the 1950s and gradual computerization has allowed railroads to eliminate many jobs.

One of those computerized services, piggybacking, has been somewhat interrupted by the constructionon Metro and the flood control project.

RF&P plans to expand its piggyback service, which now handles 6,000 trailers a month, but must wait until Metro construction is completed.

The Metro line under construction runs above ground along the river side of the Yards and begins its tunnel under Alexandria's Monroe Avenue bridge, in the middle of the Yards' piggyback facility. Because of the tunnel work, the Yards have had to relocate tracks and there has been considerable disruption of service, according to Bruce Bell, head of security at the Yards.

Work at the Yards must also compete with $50-million Four Mile Run project. The Corps of Engineers is huilding a new and wider channel for Four Mile Run where it empties into the Potomac River at the northern end of the Yards.

Much of the disruption to the Yards has stemmed from new bridges being built over the run for the George Washington Parkway, for Metro and for railroad tracks. Before the flood control project, the tracks crossed the run over a World War II-vintage trestle bridge and over 12-foot box culverts, built in the 1800s and modernized in 1906. Under the new plan, RF&P willr elocate tracks over the new bridges.

When these major projects are completed RF&P hopes to continue to increase the efficiency of the yards, says Superintendent McGinley: "The way we do business will change, but there won't be a change in the basic efficiency of a steel wheel on a steel rail."