A booster was a motor–generator (MG) set used for voltage regulation in direct current (DC) electrical power circuits.
Again in the days of direct current mains, power stations often had large lead-acid batteries for load balancing.
Sometimes one cell in the battery would become "sick" (faulty, reduced capacity) and a "milking booster" would be used to give it an additional charge and restore it to health.
The motor of the MG set was connected in parallel with the supply, usually at 600 volts, and was mechanically coupled, via a shaft with a heavy flywheel, to the generator.
Thus there was a saving in weight and cost compared to the Ward Leonard system, in which the MG set had to be equal in power rating to the traction motors.
A small "booster" transformer could be added to a set experiencing such symptoms; it would raise the voltage applied to the filament slightly, which would increase emission and restore brightness.