A railroad slug is an accessory to a diesel-electric locomotive. It has trucks with traction motors but, unlike a B unit, is unable to move about under its own power, as it does not contain a prime mover to produce electricity, and there may or may not be a cab for an operator. Instead, it is connected to a locomotive, called the mother, which provides the needed electrical power to operate the traction motors, and the motor controls.
Sorry Craig but it is not self powered
I think the distinction is the cow and calf are bought as a matching pair (A and non-powered unit) where a slug is a re purposed unit that can be tied with any unit