The Market Street subway-elevated line turns 100 years old on Sunday, and riders get the birthday gift: free trips for the afternoon.

The birth of the Market Street Line, which allowed passengers to travel easily from 69th Street to the Delaware River, linked Center City to burgeoning new development in West Philadelphia. And it helped spawn more growth west of the Schuylkill, as 69th Street Terminal sprouted in the midst of cow pastures.

Philadelphia's oldest high-speed line - which has since grown into the Market-Frankford Subway-Elevated - emerged at the dawn of intraurban rail travel, coming just a decade after the last horse-drawn car finally left the streets, following the rise of cable cars and electric trolleys. New York, Chicago and Boston already had built elevated rail lines to whisk riders above congested streets, and Philadelphia had been contemplating one since the 1890s.

Built in optimistic boom times of a city whose population was growing by 2,000 people a month, the new train line was an instant success. Within three years of its opening on March 4, 1907, the Market Street line was carrying 29 million riders a year, at a nickel a ride.

Just over six miles long, the route had 18 stations (10 elevated, 6 subterranean and two at ground level), and it cost the city $18 million to build (the equivalent of about $370 million in today's dollars). The line was operated by the Philadelphia Rapid Transit Co., which had just taken over the myriad trolley companies operating in the city and nearby suburbs.

(The Frankford extension was opened in 1922, extending the line's reach into the city's northeast.)

The rattle of trains on the elevated was the sound of progress for West Philadelphia. The area's population more than doubled, from 150,000 to 350,000, in the first two decades of the new line's operation, as factories, homes and shops rapidly replaced open farmland.

Employers like Bond Bread Co., with its bakery at 57th and Market, and Chilton Publishers, at 56th and Chestnut, depended on the El to bring workers to the job, and the El stations became magnets for businesses. Large commercial zones grew up around the stations at 52d, 60th, 69th streets.

Ridership waned after World War II, as riders turned to cars, and city residents and businesses moved to the suburbs. But the line remains SEPTA's busiest; the entire Market-Frankford line now carries about 47 million riders a year, including about 25.8 million on the Market Street portion, according to SEPTA.

Originally planned as an all-elevated route into Center City, the Market Street line was buried east of the Schuylkill because of complaints from Center City merchants. In 1955, an extension of the subway was finally completed under the river and out to 46th Street in West Philadelphia, and the line's old steel bridge over the Schuylkill was demolished the following year.

SEPTA took over the line in 1968. The transit agency is in the midst of a $567 million reconstruction of the El, scheduled for completion by 2009. The rebuilt line will allow the widening of Market Street without the elevated supports impeding traffic.

On Sunday, from 2 p.m. to 3:30 p.m., SEPTA will host a public reception in the lobby of its headquarters at 1234 Market St., with commemorative displays - including a handmade replica of the first El train - and historic photographs. And from noon to 5 p.m. on Sunday, service on the Market-Frankford line will be free.

Among those expected at the reception: SEPTA General Manager Faye Moore, Pennsylvania Reps. Harold James and James Roebuck Jr., City Councilman Frank Rizzo, state Deputy Transportation Secretary Karen Rae, Federal Transportation Administration Acting Regional Administrator Herman Shipman, and Center City District President Paul Levy.

To see a slideshow of the Market Street Elevated Line, go to http://go.philly.com/elEndText

Contact staff writer Paul Nussbaum at 215-854-4587 or pnussbaum@phillynews.com.