Items 61-79, chiefly yardwork, will be practically unaffected. Items 80 and 81 will be given their full value. The additional cost for the fuel for the added engine may be computed somewhat on the same basis as the cost of engine repairs. The fuel used by four engines will average somewhat less than that used by three engines, since the four engines will do far less work on the level and on very light grades, while on the heavier grades the engines are working to the limit of their capacity in either case. The loss of heat, due to radiation and the other causes, which are independent of the direct work done by the engine in hauling, will be the same in either case. These causes have already been discussed in § 142. It is impossible to make any general calculations as to the relative consumption of fuel in the two cases, since so much depends on the proportion of track which is level or which has a very light grade. If the four engines operating lighter trains each burn 5% less fuel than three engines operating heavier trains, we will find, by the same method as before, that the extra engine may be charged with 80% of the average fuel consumption of the other three engines. If, as before, we assume that this variation in fuel consumption is proportional to the variation in the number of trains required to handle a given traffic, then the extra engine would be responsible for 80% of the average fuel consumption, regardless of whether the number of trains saved was one in four or one in ten. Although it is true that the value 80% is a mere guess, that no general value is obtainable, and that the value for any particular and special case could only be computed with great difficulty, it is evident that the error is not very great, and we will therefore assume 80% as the fuel consumption assignable to the extra engine. The consumption of other engine-supplies, water, oil, waste, etc., is not strictly proportional to the consumption of fuel, but we will assume it to be so in this case, and that 80% of Items 83-85 are allowable for the extra engine.

Items 88 to 94, which concern train-service, will be considered as varying according to the number of train-miles, and we will therefore add 100% for all these items. Items 97 and 98 will be allowed 50%. Items 99, 101 to 103, which refer to damages, might be considered from one standpoint to be unaffected, while from other standpoints the effect might be considered as 100%. The risk of train operations varies very largely with the number of trains, and yet in some respects the danger is independent of whether there are 15 or 20 cars in a train. There will be little error in assigning 50% extra for this item. Items 104 and 105 will be allowed 100% of their net value. The general expenses are evidently unaffected. Collecting these various items we have Table XXIX.

If we assume that the average cost of a train-mile is $1.50, then the operating value per mile of saving the use of the additional engine equals 41.63% of $1.50, or 62.45 c.