In my region , I have been told the top drivers pulling Heavy loads . . steel coils 28,000 lbs, if overloaded are capable of using a maneuver . . when entering the scales of perching a back wheel on the curb . . reducing weigh in's . I was told these drivers are recruited , for these tax cheating skills , and highly paid.

Enforcement of rules, inspections, paperwork, etc. Fuel is taxed, not a toll for the road usage. In the OPs example where there is a rail line literally next to the interstate it would make sense to charge a large toll. For other highways that don't have parallel railways it would not make sense to charge a hefty freight toll.