According to the APICS CSCP - Supply Chain Management Certification, rail is a transportation mode that has the highest fixed cost and the lowest variable cost structure. Fixed costs are costs that do not vary with the volume of traffic, such as track maintenance, equipment, and terminals. Variable costs are costs that vary with the volume of traffic, such as fuel, labor, and materials. Rail has high fixed costs because it requires a large investment in infrastructure and equipment, but low variable costs because it can carry large volumes of freight over long distances with relatively low fuel consumption and labor requirements.