Thus it appears that the height of the extreme upper level in our diagrammatic series of rates is fixed by the highest charge which that particular traffic will bear.[71] Beyond a certain point, no matter how great the distance, the rate cannot be increased above this level. This maximum varies, of course, with each commodity. On cotton it may be fifty-five cents per one hundred pounds; on grain or coal it will be much lower, and on sand or cement lower still. The problem of the traffic manager is to attain this highest rate as speedily as possible with increasing distance, and to grade his rates with distance up to this level as quickly as possible, consistent, of course, with maintenance of a full volume of business. But not only may the final limit of what the traffic will bear be different for each commodity; the steps or stages by which the rate progresses up to this maximum, are quite independently determined. The actual tariffs of local class rates in general are much simpler than the commercial conditions of rate making often warrant. Probably the major portion of tonnage on American railways moves under special or commodity rates. Even in Prussia over three-fifths of the traffic is of this exceptional sort. These special rates are made with a view to particular circumstances prevalent at the time. Bids from a quarryman in Vermont on stone for a public building in Chicago, may be dependent upon the grant of a low rate on his marble in competition with a quarry in North Carolina, also able to supply the particular stone required. The various ascending series of rates are thus rendered bewilderingly complex. This is also shown by the foregoing diagram of rates between St. Paul and Chicago.[72] The rate on a cheap, heavy commodity like coal, probably rises rapidly at first, and soon attains a maximum beyond which it can never go. On this diagram, for instance, the freight rate on soft coal for points up to 180 miles out is lower than that on flour. Beyond that point the coal rate in turn exceeds that on flour. Cement is higher than lumber for the first 150 miles; but after that point the relatively greater value of lumber holds it steadily above cement. On heavy cheap commodities the relatively high cost of cartage in competition enables the railway to reap the full measure of its advantage and to charge well up to the maximum of what the traffic will bear, within a comfortably short distance. Furthermore, variable costs for terminal charges have to be considered. Wherever they are high the rate must rise at once sufficiently to cover these, no matter how short the distance; but thereafter the rate may not need to be increased greatly for some time. On light higher-grade goods the wagon is an effective competitor for longer distances.[73] Moreover, the competitive points at which rates rise from stage to stage are seldom the same for all classes of goods. A river crossing brings competition