Ask someone to picture the world's longest bridge and they will usually imagine a long span over open sea. The actual ranking looks nothing like that. The top four are all Chinese high-speed rail viaducts, and they mostly cross farmland.
The Danyang–Kunshan Grand Bridge on the Beijing–Shanghai line runs 164.8 kilometres. The Changhua–Kaohsiung Viaduct in Taiwan is 157.3 km, the Tianjin Grand Bridge 113.7 km and the Weinan Weihe Grand Bridge 79.7 km. For comparison, the Lake Pontchartrain Causeway in Louisiana — the longest bridge over open water — is 38.6 km, less than a quarter of the leader.
Why a railway would build a hundred-kilometre bridge over dry land
High-speed rail has an unforgiving requirement: the track must be almost perfectly level and must not shift. At 300 km/h, ground settlement of a few centimetres is a serious problem.
Much of eastern China is soft alluvial soil, waterlogged and prone to settling unevenly. Building a conventional embankment across it means excavating, replacing and compacting soil over enormous distances, then waiting — sometimes years — for it to consolidate before track can be laid. Even then, it continues to settle.
A viaduct sidesteps the problem. Piles are driven down to firm ground and the track sits on a concrete deck that does not care what the soil above does. It sounds more expensive, and per kilometre it is, but the comparison is not against a simple embankment — it is against an embankment plus land acquisition plus years of settlement delay plus ongoing maintenance.
The land argument is at least as important
An embankment is a wall. It divides farmland, blocks drainage and cuts local roads, so each crossing needs its own underpass or bridge. In densely farmed regions this makes the true land cost far higher than the strip the track occupies.
A viaduct on piers leaves the ground underneath usable. Farmers keep working the fields, existing roads and irrigation channels pass beneath untouched, and the amount of land actually bought is limited to the pier footings. In a country where the rail network was built quickly through intensively cultivated provinces, that calculation favoured viaducts almost everywhere.
The two American entries are a different species
The Lake Pontchartrain Causeway and the Manchac Swamp Bridge, at 38.6 km and 36.7 km, are road bridges built for the obvious reason: there was water and swamp in the way, and going around would have been longer.
That is the older logic of bridge building — cross the obstacle. The Chinese viaducts follow a newer one, where the bridge is not crossing anything in particular but is simply the most economical way to hold a railway rigid across a long distance.
A note on how these are measured
Rankings like this depend on where you draw the boundaries of a structure. A continuous viaduct with no break is straightforward, but engineers differ on whether adjoining sections built under separate contracts count as one bridge or several. Comparisons between a continuous rail viaduct and a road crossing over open water are, in a meaningful sense, comparing two different things that share a name.