The largest public-private partnership investment in a national park is delivering a new experience for visitors to the landmark Gateway Arch in St. Louis, which was designed by the late-architect Eero Saarinen. The 10-year path to the opening this month was full of hurdles, including floods. And the project’s 46,000-sq-ft underground visitor center expansion needed to be built without shutting down the arch, which gets nearly 3 million visitors each year.