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

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.