Two blocks north of Vine Street, a fissure opens in the earth to reveal a subterranean realm that might be the remains of a lost civilization. Cathedral-like tunnels pierced by shafts of sunlight give way to a dense jungle. Occasional clearings occur, offering fish-eye views of towers looming above, a welcome reminder that Center City isn't as far away as it seems.

Reading Railroad trains once roared through this urban canyon, ferrying coal to Philadelphia's industrial belt. But since the last train made the trip in 1992 - delivering giant rolls of paper into the bowels of the former Inquirer/Daily News building - nature has been busy with its own landscaping. With modest structural improvements, advocates say, the industrial relic could be repurposed as an extension of the Reading Viaduct park, scheduled to complete construction drawings next year.

The idea of creating a low-line companion to Philadelphia's planned high line has so gripped imaginations that a team of top designers has volunteered to sketch ideas for a belowground trail on the west side of Broad Street. Guided tours are now practically weekly events conducted by Paul van Meter, who first proposed a low-line park.

There's one hitch: A new city plan just earmarked the low-line trench for a high-speed bus route that would connect a string of cultural venues to the heart of downtown.

This idea, too, has gripped local imaginations. Transit advocates envision sleek electric buses slicing below street level, transporting riders from the Convention Center to the Mann Center in minutes. Because SEPTA owns the right-of-way, the line - a form of Bus Rapid Transit, or BRT - could be constructed relatively quickly and cheaply. Besides platforms, only stairs and elevators would have to be built to take riders to the surface.

So what's the best use for the low line: park or ride?

It's not every day that Philadelphians get to choose between two equally seductive visions for public improvements. One option plays to our growing fascination with our industrial past, while the other taps into our renewed interest in expanding the city's transit network.

Combining the low line, west of Broad, with the mile-long high line, east of Broad, would create a 3.7-mile belt of green space on the northern rim of Center City. Branded "Viaduct Greene," it would almost certainly ignite development in the triangle between Temple University, Northern Liberties, and Spring Garden.

An east-west bus line in the low-line trench, meanwhile, could give the city its long-sought "cultural connector," an expanded version of the discontinued 76 bus. Laura Spina, a city planner, imagines the line could someday continue to the Delaware, linking the waterfront to Fairmount Park.

Before you get too excited, keep in mind that nothing happens quickly in Philadelphia.

Right now, the park advocates have the upper hand because they could piggyback the low line onto the high-line project. After nine years of discussion, the city expects to complete construction drawings next year and start work on the first phase of that elevated park: the short, curving spur that runs from Broad and Noble Streets to the start of the main viaduct at 11th Street. Just a fifth of a mile, the ramped spur will cost $8 million, says the Center City District's Paul Levy, who is heading the effort.

The spur is merely a warm-up act for the main show: extending the park onto the main branch of the old Reading line. As the stone-and-earth trestle cleaves through the Loft District, it widens to reveal 360-degree views and an Oz-like panorama of the skyline. But the city has run into problems trying to wrest control of the main branch from its owner, the Reading Co.

In the meantime, the park on the elevated spur can't happen soon enough for the emerging Loft District, a neighborhood of old factories that has almost no public space. The original proponents, Sarah McEneaney and John Struble, were able to convince city officials that an elevated park could be made safe only after the success of New York's High Line. By renovating the spur first, they hope to build momentum for finishing the main branch of the viaduct.

Proponents of the low line see their project as a logical Phase 3, but the clock is ticking. PennDot is getting ready to reconstruct the part of Broad Street over the Reading trench, and it wants to fill the trench with dirt. That would eliminate an important entrance to a future low-line park, and interfere with the connection to the high line.

At the behest of the American Institute of Architects' Community Design Collaborative, a team of top local designers - Olin, Bohlin Cywinski Jackson, and CVM engineers - agreed to whip up a design to help make the case for the low-line park.

One of their early concepts calls for transforming the parking lot in front of the School District building into a plaza with a ramped entrance down to the low-line park. The trail could extend as far as 30th and Poplar, where the trench rises to street level.

Romantic as the idea sounds, the low line lacks some qualities that make the high line such a compelling project. The area west of Broad Street is already rich with green space. There are concerns about safety, and whether being underground would remain a fun experience once the thrill of venturing into an undiscovered ruin was gone. Proposals for food vendors and a wine bar could help maintain the allure.

Funding for maintenance is another challenge. Philadelphia can barely take care of Rittenhouse Square's seven acres, located in the most affluent part of the city. Coming up with a strategy to maintain the equally large high line in the Loft District won't be easy, and finding money for the low line would add to the burden.

Yet raising money for a bus line is even more daunting. SEPTA's strategic planner, Byron S. Comati, says the cultural connector is fourth or fifth on the agency's wish list. Right now, there isn't even funding for No. 1, an extension of Route 100 to Norristown.

In a 2006 assessment, SEPTA estimated it could build the connector for $114 million - spare change as transit projects go. Still, many wonder whether the agency could provide an even cheaper version simply by rerouting one of its existing bus routes.

In the end, it hardly seems worthwhile to wait for SEPTA. If it's going to take 20 years to realize its connector, why not use the low line as a park in the meantime? The park's stairs and elevators could always be repurposed later for the bus line.

Maybe Philadelphia doesn't have to choose, says Comati. The low line once contained four parallel railroad tracks. That's a lot of width. If and when BRT becomes a real possibility, there's no reason the bus and the skinny trail couldn't share the road.