In the context of DC motor based locos, Silicon rectifiers or Silicon-controlled rectifiers are used to convert AC voltage into DC voltage to power the traction motors. ( Conversion of AC to DC is called rectification, hence the name rectifier). In simple terms these are just switches that conduct current only in one direction ( basically DC currents) and a type of Thyristors. GTO thyristors used in the earlier WAP5 designs are also a type of Silicon rectifiers.
All the components of an electric locomotive like the transformers, starting resistances ( in case of DC motor based locos), static converter and VFD circuitry (in 3 phase locos) and mainly the traction motors generate a LOT of heat....
more... Especially for higher power locos, more power is extracted from the locomotive by pumping more current into it rather than using heavier and more powerful equipment ( as the weight is limited by the maximum bearing capacity of the track). This generates even more heat making cooling extremely necessary. Blower motors are used to cool the Traction motors and the other equipment in the locomotive.