The philosopher Heraclitus once remarked that a wise man and a fool may look at the same tree and see different things. The observation also pertains to a reasonable person and a developer.

A reasonable person, for example, would look at the Union-Pacific rail yard in downtown Sacramento and see 240 acres of dirt laced with a century of poisonous industrial byproducts. A developer looks at the same thing and sees a pedestrian-oriented urban district containing more than 4,500 units of housing and a shopping street, all oriented around a regional transportation terminal for trains and buses.

The developer’s agenda is a tall order. To begin with, the Union-Pacific site is a very large brownfield — that is, an abandoned and contaminated urban site. The developer in this case is Millennia Associates, a group that includes Los Angeles architect Jon Jerde. As an architect, Jerde is responsible for such instant-urban environments as Horton Plaza in San Diego, Universal Citywalk in Los Angeles, the Mall of America in Minnesota and the Fremont Street Experience in Las Vegas. As a developer, Jerde last year completed a four-block residential-and-retail project in Salt Lake City. In November, the developers entered talks with Sacramento city officials on an exclusive-right-to-negotiate basis. Millennia and the city are expected to come up with a development plan some time this year.

To date, the Millennia Associates plan is still sketchy because the developers want to meet with community groups before finalizing their development plans. The current site plan, therefore, is little more than a land-use map, with a new street grid and different development types identified. Still, the Millennia proposal is remarkable in at least three ways. The first is density: As mentioned above, the developers want to build about 4,500 dwelling units, including single-family homes, townhouses, apartments and condominiums. Essentially, the entire scheme could be described as a large-scale, transit-oriented development, with most of the shopping and transit connections within easy walking distance of housing.

The notion of building housing on a contaminated site is notable in itself. In the past, housing and schools required a pristine level of clean up while commercial development was held to a lower standard. Not surprisingly, the typical brownfield project is an industrial or retail use, such as offices and big-box retail centers, but rarely housing. This year, a new rule from the state Department of Toxic Substances Control allows homebuilding on less-than-pristine land, if the developer constructs the housing atop a podium, i.e. the housing is built atop a parking garage.

The advantage of podium housing is that it is versatile. The downsides are that landscaping options are severely limited on the hard surface of the podium floor and the streetscape can be uninviting. Ambitiously, the Millennia plan also calls for public parks on terra firma, which will be dug out and replaced with clean fill.

The second remarkable part of the design is the proposal for a rehabilitated train depot that will be expanded significantly to include new platforms for both passenger trains and local commuter rail, as well as buses. Preservationists waged a battle with transit boosters, who wanted to demolish the old train station in favor of a modern, multi-modal facility. The current plan tries to accomplish a happy compromise by making the historic building the centerpiece of a much-expanded transportation center. Among the many things that are on the table are ways to finance a $160 million expansion of the historic train depot on the site. Recently, the same parties have been discussing the feasibility of moving the old rail station 400 feet(!), presumably to make it easier to build the new facilities. I predict that the cost of this adventure, as yet unknown, together with likely damage to the building will cause this brainstorm to lose its thunder.

Architecturally, the most ambitious part of the plan would be to make Fifth Street into a elevated bridge that flies over the train tracks, while serving as a shopping street. (The developer likens this idea to the Ponte Vecchio, the bridge in Florence, Italy, that spans the Arno River and is lined with shops on either side.) The concept of a shopping bridge is vintage Jerde, a man with a bold imagination and long experience with retail design. If the idea were proposed by anyone other than Jerde, in fact, I would be more skeptical. But Jerde has the panache to carry off the design. Still, it is strange kind of urbanism.

Another big plan for the Union Pacific site is to build a new basketball arena for the Kings. (Although this is not part of the Millennia proposal, the developers say they could accommodate the arena, if necessary, although it would take away about 20% of the housing.) Without more detailed plans, it is impossible for me to say much about the arena plan, which has been the source of much controversy in Sacramento. Of course, the success of downtown sports stadiums in Baltimore, Denver and other cities has made this idea fashionable. But very large buildings like arenas and stadiums do not harmonize easily with the kind of low-rise neighborhood envisioned by Millennia. On the other hand, the presence of the transit hub would be a strong rationale for locating the arena in the old train yards.

As ambitious as plans are for the Union Pacific site, they seem almost small compared with an even larger proposal to the immediate north: The creation an 800-acre park, or something just slightly smaller than Central Park, in the delta where the American and Sacramento rivers meet. Some people claim that housing densities would have to triple in the Union Pacific site to accomplish both the city’s residential goals and the park. But a magnificent park near the water would be a worthwhile amenity for a city that could use the open space. Fools like me find it difficult to envision anything other than real estate on the northern edge of downtown Sacramento. Maybe there are enough wise men in the city who can envision something more.