The Rodin Museum is a beloved Philadelphia icon, a taut little neoclassical temple designed for the Parkway by one of the city's master architects, Paul Philippe Cret. The City Branch rail bed is a cultural treasure of a very different sort. Some envision the exuberantly overgrown right-of-way as a ribbon park that could link the Reading Viaduct to the Art Museum.

The two have sat side-by-side for nearly a century - both artifacts from the city's manufacturing heyday - mostly ignoring each other. But now their fates have become intertwined by a proposal for an apartment house over a one-block section of the rail bed, which runs in an open trench behind the museum. The building's south facade would be just 60 feet from the Rodin's back door.

It is a project that manages to be at once both dismaying and intriguing. Commissioned by David Blumenfeld's Cross Properties, the mixed-used building promises to fill in one of the remaining gaps in the booming neighborhood north of the Parkway. Yet the six-story apartment house would drastically alter how we experience these two important historic structures.

The case is more difficult than usual because Cret's museum is among Philadelphia's most recognizable works of architecture. It derives its charm from the serene and aristocratic way it resides on its Parkway site, surrounded by greenery like an isolated country villa. While there are many high-rises nearby, the specific location of Blumenfeld's building could put an end to the fantasy and make the diminutive Rodin look hemmed in by a giant.

The impact on the jungly landscape of the rail bed, which has been dubbed the "low line" by the group that wants to turn it into a trail park, could be equally profound.

Two stories below street level, the trench also benefits from the perception of isolation. Walking its two-mile length, you experience the city at a distance, occasionally glimpsing snippets of the skyline above its massive stone walls. Once capped by the apartment building, the pit behind the Rodin would be reduced, at best, to a dim tunnel. At worst, the corridor would be cut into two useless pieces, rendering the park idea stillborn.

For all that, Blumenfeld's proposal does offer the city something in return.

The building would immediately establish a strong urban presence on Hamilton Street, between 21st and 22d Street. It would be part of a growing line of grand residences stretching from Pennsylvania Avenue - where Cret built his last project at No. 2601 - to the new Granary Apartments on Callowhill Street, behind the Free Library.

Since the Barnes Foundation opened in 2012, this area has become a magnet for developers. Another residential project, Rodin Square, just began construction on Hamilton Street, directly across from the trench. Along with a pair of 10-story apartment houses, that complex will feature a new Whole Foods and other retail.

Blumenfeld also plans a row of storefronts on Hamilton Street. That will cement the block as a retail hub. He also wants to tuck the parking garage underground. At the same time, he will preserve a corridor for a future rail line, as required by a SEPTA easement. The park is what could be the big loser.

Still, as wonderful as a trail could be, the project remains a far-off dream - the city hasn't even been able to pull off the more manageable high-line park on the Reading Viaduct. One argument against using the two-mile-long, submerged corridor for a park is that it would compete for users and money with the parallel public space of the Parkway.

The real issue is whether Blumenfeld's architects, Barton Partners, can create something worthy of Cret's Rodin.

So far, the answer is no. To be fair, its design is still in an early stage. Barton has simply sketched a site plan showing the size and arrangement of the building's parts. It hasn't colored in the fine details or selected materials, the magical alchemy that produces architecture. Blumenfeld (brother of Divine Lorraine developer Eric Blumenfeld) says he wants to get the support of the Logan Square Neighborhood Association before paying for a more detailed design.

That group seems mostly concerned with traffic at the already congested intersection of 21st and Hamilton, rather than design. But for the sake of the city's iconic Rodin Museum, design needs to be the priority.

Right now, the architects are showing a building shaped like the number '7.' A long, curving facade would face the back of the Rodin, while the retail portion on Hamilton Street would be just one story. Blumenfeld has promised to install a 60-foot wide linear park between the curved facade and the Rodin, with a restaurant opening onto the green space.

It would be better for the Rodin if the designers flipped this layout, placing the long facade on Hamilton Street. That would give the museum the breathing room it deserves, without compromising the Hamilton Street retail. The other benefit is that a narrower, more compact structure could also preserve space below for a future rail park. The designers could even embed light wells in the strip park to make the tunnel feel less claustrophobic.

Reconfiguring the site plan is fairly easy. Creating architecture that honors the Rodin is much harder.

Based on their portfolios, neither Barton nor Blumenfeld has ever designed anything approaching what is needed here. In the last few months, Barton has produced some smart urban site plans for Philadelphia developments that never materialized. But its completed residential buildings have been the lowest form of developer-driven architecture, bloated in scale and poorly detailed.

To realize a building on this site, Blumenfeld will be asking the city for a long list of favors, including a zoning change and an exemption from a provision in the building code. It is time for the city to demand something meaningful in return: a work of architecture that can stand proudly next to Cret's masterpiece.

Changing Skyline: