A viaduct is a long bridge carried on a row of tall legs or arches, built to keep a road or a railway level while the ground below drops away. Think of a train line crossing a wide valley: instead of going down one side and up the other, engineers build a viaduct straight across, and the train stays flat. The key idea is the row of supports. An ordinary bridge often crosses in one span; a viaduct marches across on many pillars, one after another, and it is usually long and high. Old ones in Britain are beautiful brick arches; new ones are plain concrete. It does not have to cross water — that is the main difference from most bridges people picture. A viaduct can cross a dry valley, a town, farmland, or another road. Taiwan's high speed rail runs on viaduct for most of its length, above the fields of the western plain, though in Chinese we normally call that 高架橋. The word is a noun, countable: a viaduct, two viaducts. It feels a little formal or technical — engineers, news reports and travel writing use it. In everyday talk people just say bridge, or 高架.