第三軌条方式
Third rail railways predominantly operates in urban areas. Notable exceptions are (or were) mainline electrics of the former Southern Region of British Railways and a few interurban railways in the US. In Europe top contact third rail tends to be limited to early electrified urban railways (the current conductor is normally left naked on top), contrary to North America where it usually has a protecting cover. Considered safe, the covered top contact conductor also appeared at most North American systems built relatively recently. Modern European systems predominantly make use of bottom or side contact power rails.