Booster transformer is a device which is often used toward the end of power line in order to raise the voltage to the desired value.
According to Wikipedia
A buck–boost transformer is a type of transformer used to make adjustments to the voltage applied to alternating current equipment. Buck–boost connections are used in several places such as uninterruptible power supply (UPS) units for computers and in the tanning bed industry.
A booster transformer is used in case of making the voltage constant to the load as well as it collect the return current from the earth and from the centenary conductor and supply to the locomotive engine . A booster transformer is also used in the UPS , to make the output voltage constant it is used .