China completed the Danyang-Kunshan Grand Bridge in 2010, an absolutely massive piece of high-speed rail infrastructure that stretches for an incomprehensible one hundred and two miles between Shanghai and Nanjing.

Why elevate a railway across a plain?

The Danyang–Kunshan Grand Bridge is not one enormous span over open water. It is a viaduct: a long, raised railway supported at intervals by piers. That design suits the low, wet landscape of the Yangtze River Delta, where fields, waterways, roads, and built-up areas are interwoven. A ground-level route would have needed to detour around many of them or cross them with repeated embankments and bridges.

The bridge forms part of the Beijing–Shanghai high-speed railway. Its name describes the stretch between Danyang and Kunshan; it is not the entire rail link between Shanghai and Nanjing.

A continuous track needs a predictable path

High-speed trains work best on track with gentle slopes and broad curves. Raising the line lets engineers carry it over local obstacles while keeping the railway’s overall profile relatively even. The track still bends where the route requires it—the bridge is not literally a perfectly straight line—but it avoids the stop-start geography that would come from following every patch of dry ground.

The supports also keep the railway above much of the surrounding lowland. Instead of building a solid raised bank across the landscape, engineers can leave space between the piers for water and other uses of the land. The result is a route that occupies a narrow corridor overhead rather than a continuous wall at ground level.

Building in repeating pieces

A project this long depends on repetition. Many similar spans can be constructed and joined into a continuous structure, while the piers transfer the weight of the track and trains down into the ground. That does not make the terrain irrelevant: foundations must be suited to local soil, and the finished railway must remain stable enough for fast trains.

Completed in 2010, the bridge shows the practical bargain behind high-speed rail across a flat but complicated region: build a great deal of elevated infrastructure once, so trains can cross a patchwork of fields and waterways without turning each obstacle into a separate route change.