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. There's one hitch: A