A traction substation, traction current converter plant, rectifier station or traction power substation (TPSS) is an electrical substation that converts electric power from the form provided by the electrical power industry or railway owned traction power network to an appropriate voltage, current type and frequency to supply trains, trams (streetcars) or trolleybuses with traction current. A traction power substation may also refer to a site that supplies a railway traction power network with power from the public electricity utility.

The exact functions and power conversions made by a traction substation depends on the type of electrification system in use. Broadly there are three categories of electrification system each with different system architectures: Low Voltage DC Electrification (using conductor rail of overhead line), High Voltage Low Frequency AC Electrification with overhead line, and 25kV mains frequency AC with overhead line.

However, across all systems, traction substations can be defined as any site where multiple overhead line or conductor rail circuits are connected to through (usually) circuit breakers to a busbar. This allows for two functions.

Beyond this, many substations will have additional functions for providing new power to the electrification circuits (from grid connections, other high voltage cables, a stepdown transformer or transformer with rectifier) or for separating different supplies apart from each other by way of circuit breakers fitted between sections of the busbar that are normally open (from now on "normally open points").

But at their core all substations will at least perform those initial two functions, and will revert to that basic state in degraded operations, such as if a particular supply or step-down transformer at a specific site falls offline. Because of these two basic principles, substations are often located at or near major stations and junctions between multiple electrified lines so as to allow for flexibility in switching and feeding arrangements. But also for regular substations to be spaced across long distances of plain line to reduce volt drop.

In terms of busbar configuration, railway substations are almost always radial, the simplest type.

Low voltage DC systems were the first kind of electrification to emerge in the late 1800s using both overhead line and conductor rail collection. Although many countries (especially in Japan and across Europe) implemented 1.5 kV or 3 kV DC overhead line networks across mixed traffic mainline networks, the system is mostly deployed nowadays on rapid transit, or short distance/high frequency suburban systems as well as light rail, trams and trolleybuses. 750 V is the standard for metros using conductor rail collection and tram or light rail (using overhead line) while 1.5 kV is the standard for metros and commuter systems using overhead line collection.

The main benefit of these systems is the relative lack of on-board conversion equipment required by the train with it possible to connect the traction current directly to the traction motor control equipment. This is opposed to AC electrification which requires voltage transformation and (typically) rectification on board every train. However, the lower line voltage means a greater number of tractions substations must be built with the DC current needing to be supported at regular intervals of between 2 and 12 km depending on exact system voltage and traffic levels/line speed. Although a higher line voltage would decrease the number of substations needed, the maximum traction voltage of a DC system is limited by