With the packet of envelopes in his hand, Allan descended the stair and came out upon the grimy platform. Just across the yards lay the low, dark, brick building which was the freight office, and he made his way toward it over the tangle of tracks and switches, where the freight-trains were being "made up" to be sent east or west. After some inquiry, he found the freight agent gazing ruefully at a barrel of oil which had just been smashed to pieces by a too vigorous freight-handler. Allan gave him the letters addressed to him and hurried away to deliver the others. Farther down the yards was the office of the yardmaster, a little, square, frame building, standing like an island amid the ocean of tracks which surrounded it. Here was kept the record of every car which entered or left the yards--the road it belonged to, its number, whence it came, whither it went, by what train, at what hour. This dingy little building was one link in that great chain of offices which enables every road in the country to keep track of the cars it is using, to know where they are, what progress they are making, and what service they are performing. Every one who has seen a freight-train has noticed that it is almost always composed of cars belonging to many different roads, and must have wondered how these cars were kept accounted for. Every road would prefer to use only its own cars, and to keep them on its own system, but this is impossible. A car of sugar, for instance, sent from New York to Denver, must pass over at least two different lines. It can go from New York to Chicago over the New York Central, and from Chicago to Denver over the Santa Fé. Now, if the car belonging to the New York Central in which the sugar was loaded at New York be stopped at Chicago, the sugar must be reloaded into another car belonging to the Santa Fé, a long and expensive process to which neither the shipper nor the road would agree. To avoid this loading and unloading, freight in car-load lots is always sent through to its destination without change, no matter how many roads the car must traverse, and when it reaches its destination and is emptied, it is usually held until it can be loaded again before it is sent back whence it came. When the traffic is not evenly balanced,--when there is more freight, that is, being sent one way than another,--the "empties" must be hauled back, and as "empties" produce no revenue, this is a dead expense which cuts deeply into the earnings. The roads which use a car must pay the road which owns it a fee of fifty cents for every day they keep it in their possession, whether loaded or empty; hence the road holding it tries to keep it moving, and when business is slack and it is not needed, gets it back to its owner as quickly as possible. If it is damaged in an accident on a strange road, it must be repaired before it is returned to its owner; if it is totally destroyed, it must be paid for.