Since a large portion of a railroad's expense goes on from day to day irrespective of the amount of its traffic, and since the same equipment is used to haul a variety of products, railroads find it profitable to do some of their business at a loss. Let us suppose that a certain line operates nine trains a day at a total cost of $4500, or $500 a train. According to our supposition, this cost includes everything - operating expenses, interest on capital, and replacement fund. Let us suppose further that the entire additional cost involved in adding a tenth train would be $300, making the total cost for operating the ten trains $4800, or $480 a train. Can the railroad under these conditions afford to operate the tenth train if it earns less than $480? It can. Anything above the $300 extra cost involved in its operation would be clear earnings.