My guess is experimental aircraft; something like magnetically charged poker chips in an electro-magnetic field. In theory they could travel nearly to the speed of light within an electromagnetic field. One might use the Earth’s magnetic field or create a field that could be aimed at the craft’s line of travel. Who can say? There should be innumerable methods of generating charged e.m.g.fields for charged space chips and pucks to fly through.
Aiming electromagnetic fields, or creating e.m.g. field ‘bridges’ between remote planets- possibly converting the sun’s photons emissions for power, would provide highways in micro-gravity for charged space chips to travel in. It might be feasible to create simple moon-girdling charged fields for human transport pucks to travel quickly through too.