A heavy locomotive costs considerably more than a light locomotive. A heavy locomotive costs more to operate, will burn more fuel, use up more water, lubricants, and other supplies. The heavier engine will cause more damage to the road-bed and will produce greater wear on the rails and ties. On the other hand, the wages paid to the enginemen, although somewhat greater on heavier locomotives, do not increase in proportion to the tonnage. The hauling capacity of the engine is far greater and the operating cost per ton-mile is far less. The introduction of air-brakes on all passenger-trains and on freight-trains to a sufficient extent to permit the engineer to handle the train, without the use of brakemen working hand-brakes, reduces the proportion of brakemen per car, or, in other words, increases the number of cars to one brakeman, and therefore decreases the cost per car or per ton-mile for train service. It is comparatively easy to demonstrate that the cost per ton-mile of handling the trains by means of heavy engines is less than the cost when using light engines. The precise determination of such economy is far more difficult, and must of course be considered to some extent uncertain. It is not a very difficult matter on any one road to determine the relative cost of operating two types of engines, one of which can haul 60 to 70% or even 100% more cars than another. But even after such calculations are made the results have only a general value, since they apply only to those two types of engines. Of course such comparative figures will have their value to the railroad manager who is considering the policy of renewing or substituting for his old light-weight locomotives a far heavier type of locomotive, and who wishes to justify the additional expenditure for the heavier locomotives by a demonstration of the economy that would be obtained by using it. Perhaps the most simple method of making the calculation is to consider the value of the substitution on the basis of the number of trains that would be thereby saved in handling a given amount of traffic. It may readily be appreciated that the gross amount received by a railroad company for handling a certain gross tonnage of freight is a perfectly definite figure, which is neither increased nor diminished whether it is handled in four, six, or eight train-loads. If the manager can demonstrate that by the substitution of heavier locomotives a certain gross tonnage of freight can be handled in one or two less trains than would be required by the lighter locomotives, it is only necessary to compute the saving by running a loss number of heavier trains, and then he has a basis on which to determine the justification of the added cost of his heavier locomotives.
The general method adopted in this connection will be to consider the percentages of the various items of cost of a train-mile as given in Chapter VI, and to estimate the effect of the special circumstances of the problem on each one of these subitems. If the relative power of the two types of locomotives considered is such that the heavier locomotives can haul in three trains as many loaded cars as those of the lighter type would handle in four trains, or if the heavier engines handle in four trains as many cars as would require five trains with lighter engines, then the use of the heavier engines will save one train in four or one train in five, as the case may be. We will therefore compute the added cost of running the extra train. We may also consider this cost to be the same as the saving by avoiding the extra train, but, since the number of cars remains the same, we may consider that the total added cost is the cost of running say four engines instead of three. Of course, this does not mean the cost of running the additional engine "light." In the case here considered the gross tonnage of freight to be handled is assumed to be constant. We will therefore assume that the total number of cars used in handling the freight is identical. We may therefore assume that the effect of these cars on the wear and tear of the track (Item 6) will be constant. It has been estimated that locomotives are responsible for approximately 50% of the general track-wear, on account of the greater concentration of loads on the driving-wheels. In recent years the wheel loads of the heaviest freight-cars are as great as, or greater than, the driving-wheel loads of a few years ago, but the driving-wheel loads have also increased, although not in as great proportion. Although the disparity between driving-wheel loads and car-wheel loads is not now as great as it was twenty years ago, it is still sufficiently great, so that we may assume that 50% of the damage is the result of the wear due to locomotives.