I'm no friend of trucks, but I wanted to clarify that 80,000 is the typical maximum weight allowed for a semi-truck. That would more likely be a shorter-haul truck moving gravel or other materials instead of less dense cargo like Walmart products. For the long-haul, materials are transported by train.
While these road taxes are an interesting dimension, the main reasons Walmart's products are shipped via truck is because they don't want their own restocking schedules limited by train schedules. If efficiency were to dictate their logistics, the large Walmart regional warehouses would be located on a rail line and trucks would distribute the short haul from the regional warehouse to each store. Oh, well.
To reiterate, rail line maintenance expenses are not pushing Walmart cargo onto trucks. If those fees were so high, low-margin materials like gravel and sand would be in trucks and not hauled via train across multiple states.