“Powerfully primed movers” I think you mean “powerful prime movers;” the term refers to the vehicle being the primary (in these cases only) source of power for the entire road train. The term is also used in the railroad industry to refer to the actual diesel engine that provides the power to generate the electricity that runs the motors on the axles. The Caterpillar 798AC IS NOT an electric vehicle, it has a 3,500 Cat C175-16 diesel engine (5,187 cubic inch displacement) which turns a generator which provides electricity to electric motors at each wheel, just like every diesel railroad locomotive on earth.