The position of the axles of a traffic object can be defined in the Traffic Object Template Editor by setting the front and rear overhang relative to the bounding box of the vehicle.
The default overhang is 1/6th of the length of the the traffic object.

This corresponds to the following Infofile Key in the Traffic Template File:

Motion.Overhang =  value_front value_rear

(Refer to section 22.4.4 of the CarMaker Reference Manual.)