Axle loading on locos can be kept down with body mounted traction motors, eg class 91.
I think you mean unsprung mass, not axle loading. One way or another, all the mass on the loco finds its way through the axles to the point of contact with the rail.
The issue with unsprung mass is that it is just that - unsprung, and as such puts shock vertical forces into the infrastructure, which are much more severe than all the mass that is above the springs and the dampers.
In the past, as in the case of the Class 91 loco, the unsprung mass was reduced by mounting the traction motors on the body and transmitted to the wheels via Cardan shafts and final drives.
Modern locos (and some MUs, including, I think the Voyagers / Meridians) generally incorporate a "hollow axle" system, which produces a similar outcome, but all the drive system is bogie-mounted.
To keep the total mass of the
Polaris electro-diesel power car down to a target of 64 tonnes, (resulting in an axle load of 16 tonnes, assuming even weight distribution can be achieved) but provide multiple unit type acceleration, traction motors will be located on the power car and the first trailer car, adjacent to it.