There is a very good reason why traction motors are not mounted directly on axles. It is called "un-sprung weight." The truck frame is "sprung," meaning it rides on springs between itself and the axles. The axles and wheels do not get to ride on any springs. They must take the shock and pounding of all irregularities in the track. Because of this pounding, a traction motor mounted on the axle would be shaken apart and would have a very short life. The amount of un-sprung weight must always be kept to a minimum in order to easily follow the irregularities in the track. It is the same with the wheels, tires and axles of highway vehicles.