Seems great for indoor use such as switching a car shop or manufacturing plant, and thing around a painting or high dust area, but the main draw backs would be the same as for cars, with the exception of weight, which is often a good thing for locomotives. If you have to drag the weight of the batteries in a separate car you give up the advantage of tractive effort created by all that mass sitting on top of the driving wheels. Regenerative braking is great charge extender, but you still need to idle the machine while it charges somewhere. A buzz box type arrangement in the middle of a diesel consist that charges from the other locomotives when not having power drawn from the batteries would make and interesting hybrid that could help with acceleration and hill climbs, but would require extensive modifications to the coupled diesel(s).
I think its going to be a while until this goes from pipe dream to Tesla status. although since I work at a stationary power plant - bring it on! And all the electric cars you can charge too. Even better charge them at night so the grid demand profile even out.
