Demurrage shall be assessed by the serving rail carrier, i.e., the rail carrier providing rail cars to a shipper at an origin point or delivering them to a receiver at an end-point or intermediate destination. A serving carrier and its customers (including those to which it delivers rail cars at origin or destination) may enter into contracts pertaining to demurrage, but in the absence of such contracts, demurrage will be governed by the demurrage tariff of the serving carrier.