By traction current one understands the electric current, which is used for the drive of electrical trains. The current is usually supplied to the vehicles by way of an overhead line with current collectors, or by means of a third rail.

Historically some different current systems developed in different countries or at different railway companies. The world-wide largest spreading has alternating current with a tension of 25 kV at 50 cycles per second. Besides there are railways, which drive with alternating current from 50kV also in the USA (Lake Powell Railway) and South Africa. In the German-speaking countries alternating current with 15 kV and s frequency of 16 2/3 cycles per second (today 16.7 cycles per second) dominates. This system is used also in Norway and Sweden. An AC current with 25 cycles per second is used in the USA by some courses in the area from New York and in Austria by the railway of Mariazell. In numerous countries as for example Italy, Japan, Poland or Russia most electrical trains run with direct current with a tension from 1,5 kV or to 3 kV. Three-phase alternating current was used until 1976 in north Italy for electric trains. It is only used today because of the complex at least two-pole overhead line with at few railways. The most important railways still using three-phase alternating current are the Jungfrau railway and the Gornergrat railway in Switzerland.

Because traction current, in particular for trains, which drive with single phase change AC (16 2/3 cycles per second or 25 cycles per second), usually cannot be taken easily from the public electricity grid of the country, in areas, in which trains run with a single phase AC of decreased frequency, separate traction power networks are maintained with own traction current lines. In a substation this current is then transformed down for the supply of individual distance sections on the overhead line voltage. In traction current converts and static inverters for traction current the current of public grid is converted converted to traction current (and vice versa).