Railroad expenditures, as Taussig clearly pointed out a number of years ago,[44] afford a prime illustration of the production of several commodities by a single great plant simultaneously at joint and indistinguishable cost. The classic economists illustrated this law by the joint production of wool and mutton and of gas and coke. In both of these instances neither commodity could conceivably be produced alone. Nor was either one, so to speak, a by-product of the other. So nearly of equal importance are the two, in fact, that the cost of production for each may approximately be determined by dividing the total cost according to the relative worths of the two or more products. The law of joint cost with reference to the production of transportation is somewhat different. Compare, for instance, the carriage by a railroad of thousands of passengers and different commodities in every direction, under varying conditions, singly or by wholesale, slowly or by express, over a given set of rails every day; with the operation of a great refinery producing simultaneously kerosene, gasoline, lubricating oils and greases as well as various odd chemicals. Both are examples of production at joint cost, but with various important contrasts. In the refinery all the costs are joint. All the processes are interlocked. Every increase in the output of kerosene produces _pari passu_ an increase of the other commodities. On the railroad not all, but only a part of the costs are joint, in such manner as has been shown. For, from the joint portion of its plant--roadway rails and locomotives--the railroad may produce transportation of different sorts quite independently. It may choose to especially cultivate its passenger traffic, or its cotton or coal business. After a certain point of congestion is reached, the various sorts of traffic on the railroad may even become actually competitive with one another so far as the joint use of the plant is concerned. It is plain that this could never happen in the refinery. The use of more stills for making kerosene would automatically produce more by-products of every sort. But on a railroad it might well happen that the coal and passenger business might come to interfere with one another. A choice of emphasis as between fast refrigerator beef or fruit traffic, and limited express service, may have to be made on a long single track line. Nevertheless, in spite of these peculiarities of transportation, the general law of joint costs holds good, in that it is a demand for each service rather than its cost which finally determines the chargeable rate.[45] This must be so, because of the fact that the cost of each shipment is so largely joint and indeterminate, and that a large part of the entire plant is indistinguishably devoted to the general production of transportation without reference to particular units of business. One concrete example may serve to illustrate this point.