Up to this point it has appeared as if, in making distinction between the constant and variable expenditures of a railroad, it was the latter only which grew as the volume of traffic increased. This is not absolutely, but only relatively true, not only of the so-called constant operating expenses, but of fixed charges as well.[39] Everything depends upon the length of time under consideration. Many expenses follow the fluctuations of business, not evenly but by jerks. Up to the full limit of utilization of the existing plant, each increment of traffic seems to necessitate but a very small increase in the so-called variable expenses, with hardly any change at all in the constant ones. A branch road can haul more and more tons of freight with a given outfit of cars and locomotives by merely increasing slightly its outlay for fuel, train service, wages and supplies. But after a certain point more rolling stock must be provided to accommodate the growing business. As each of these additions to property occur, they contribute new quotas to the fixed charges and to the so-called constant expenses of operation, such as maintenance of roadway and the like. Nor can these new expenses be allocated to the new business alone. The moment the old traffic has outgrown the existing plant, the new expenditure becomes chargeable to all the business alike. The new outgo must be distributed evenly over the entire volume of traffic thereafter handled. Each ton, both of old and of new traffic, beyond the haulage capacity of the locomotives then in service, is equally responsible for the expense of new equipment purchased. Although the old business could have been handled without a million dollars spent for double-tracking or terminal enlargement, this addition to the expense of maintenance of way or to the fixed charges is equally attributable to every ton of traffic hauled.