Car capacity, both as regards ability to load and carry economically, is the principal factor in the determination of minimum carload rates. It is largely a question of relative cost of operation.[369] Reference has already been made to the great economy incident to the use of large cars, whereby the paying load becomes less in proportion to the deadweight. This, of course, largely accounts for the steady increase in carload capacity in recent years. But the question is even more complicated. An adjustment must be made between two main groups of freight: first, that which is sufficiently heavy to be readily loaded to the minimum weight in ordinary cars; and, secondly, light and bulky goods of which the common car will contain but a small proportion in bulk of its truck capacity by weight. Fortunately, we may evade the moot point, theoretically, as to whether a carrier is entitled to the same revenue from a given vehicle, whether it be loaded with heavy or light goods; that is to say, whether the rate ought properly to decrease per pound with increase in the density of the lading. This is a technical matter as to cost. But it carries certain implications of considerable importance commercially, as will shortly appear. The difficulty of conforming carload minima upon light and bulky articles to those on heavier goods has appeared with each attempt to standardize equipment. Widely divergent rules in the three main classification territories still cause great confusion in this regard. There is a constant temptation to construct extra long or wide cars, particularly in the western states, in order to assist the manufacturers of such light and bulky products as furniture and agricultural machinery in their competition with dealers in the East, shipping under Official Classification requirements. In other words, the penalty carried under the rules as to minimum carloads, for the use of cars larger than the standard, has been much less in the West than in the East and South. The situation has been further complicated in some instances by the arbitrary action of state railway commissions. The experience in this regard is illuminating, as again showing the extreme delicacy of adjustment in such matters under the stress of commercial competition. The short-line distance between the Missouri and Mississippi rivers lies entirely within the state of Missouri. It governs, as we have already seen,[370] the entire rate structure in this part of the country. This commonwealth some years ago by law fixed a carload minimum of 20,000 pounds for furniture, agricultural implements and wagons.[371] As it is not practicable to attain this minimum load on an ordinary standard car, the Missouri shipper was stimulated to demand larger equipment in order that he might avail himself of the lower rate for carload lots. The local railways, accordingly, built such cars, which, of course, travelled far beyond the limits of this single commonwealth. This forced other