Well, I imagine the German modelers managed to craft 2-rail finescale power trucks somewhere.
Three-rail trucks would be magical. In my hypothetical empire, light-rail/trolley routes would have a physical connection to the rest of the layout somewhere, so I'd have to have some means of 3R traction (I could cobble together an assembly for non-powered bogies), to warrant pursuing the acquisition of some of these cars for the purposes of effecting a powered conversion. Especially the newest DMU's as the prototypes do run in mixed service, as far as I know.
---PCJ