Railway electrification using at are used on transport railways in Germany, Austria, Switzerland, Sweden, and Norway. The high voltage enables high power transmission with the lower frequency reducing the losses of the traction motors that were available at the beginning of the 20th century. Railway electrification in late 20th century tends to use AC systems which has become the preferred standard for new railway electrifications but extensions of the existing networks are not completely unlikely. In particular, the Gotthard Base Tunnel (opened on 1 June 2016) still uses 15 kV, 16.7 Hz electrification. Due to high conversion costs, it is unlikely that existing systems will be converted to despite the fact that this would reduce the weight of the on-board step-down transformers to one third that of the present devices. History The first electrified railways used series-wound DC motors, first at 600 V and then 1,500 V. Areas with 3 kV DC catenaries (primarily in Eastern Europe) used two 1,500 V DC motors in series. But even at 3 kV, the current needed to power a heavy train (particularly in rural and mountainous areas) can be excessive. Although increasing the transmission voltage decreases the current and associated resistive losses for a given power, insulation limits make higher voltage traction motors impractical. Transformers on each locomotive are thus required to step high transmission voltages down to practical motor operating voltages. Before the development of suitable ways to efficiently transform DC currents through power electronics, efficient transformers strictly required alternating current (AC); thus high voltage electrified railways adopted AC along with the electric power distribution system (see War of the currents). The 50 Hz (60 Hz in North America) AC grid was already established at the beginning of the 20th century. Although series-wound motors can in principle run on AC as well as DC (the reason they are also known as universal motors) large series-wound traction motors had problems with such high frequencies. High inductive reactance of the motor windings caused commutator flashover problems and the non-laminated magnetic pole-pieces originally designed for DC exhibited excessive eddy current losses. Using a lower AC frequency alleviated both problems. In the German-speaking countries, high-voltage electrification began at , exactly one third of the national power grid frequency of 50 Hz. This facilitated the operation of rotary converters from the grid frequency and allowed dedicated railway power generators to operate at the same shaft speed as