Electrified tandem trailer axles?

Here's a concept for you: replacing a trailer's "passive" axle with one powered by an hybrid electric package that helps capture "wasted energy" during nominal operating conditions, such as when braking, stores that energy in a lithium battery pack, and then feeds it back into an electric motor to provide extra "oomph" for getting up hills, etc. End result: an average of 30% fuel savings for a tractor-trailer, at least according to this unusual trailer axle package manufacturer, Hyliion. (All photos by Sean Kilcarr/Fleet Owner)