Even prior to the steam locomotive, railed roads faced a peculiar problem. For example, a coal car would be loaded at a mine and then conveyed to the coal consumer. Upon delivery the freight charge was paid and the railroad was left with an empty car that needed to be returned to the mine for the next load, a non-revenue producing movement. So, the tariff had to reflect the fact that the costs of moving the load of coal were not simply in one direction. This issue is called backhaul, and many forms of transportation are faced with this problem.
Inevitably, lots of empty cars would end up at one location on the railroad and there would suddenly emerge a dire need for empty cars at another location. This train could be heard for miles before it finally arrived since there were no loads inside the cars to dampen the rattling of metal cars bouncing on metal rails. As train movements go, a load of empty cars was of lowest priority, taking the siding for every other train which was producing revenue for the railroad. If the shipper really needed fresh cars, then the train’s priority moved up bit, but backhaul movements are simply an unfortunate necessity.
The railroad benefited if a car type could be used for a variety of loads. By having a car which could be used for a variety of loads, back haul was lessened. As railroad technology spread, specialized car types emerged. And the more specialized that the car was, the more likely that it would be traveling empty at least half the time. In some cases, these cars were used infrequently, but when they were needed, there was no other car like it. Such cars would be designed for a specific purpose and, again, the tariff would reflect this. These cars were varied and interesting; some cars were created for special freight traffic. Cars which haul food products need to be kept immaculately clean and tight to prevent spoilage. Others, such as those used in cow hide service, were so foul as to be unusable for any other purpose.
For livestock cars, transport revolved around the needs of the meat packing plants. In the farming regions, vast numbers of cars were needed for short periods of time when the crops were harvested. And when the freight had been delivered, these cars would have to be stored at an area near the potential freight loading until future use. In general, open hoppers carried commodities which were not affected by the weather. Others were in specialized service, such as cement and silica, which needed to remain dry. Yet the inevitability was that these cars were often traveling empty. This was exacerbated by Interstate Commerce Commission rules which required that a car belonging to a specific railroad needed to be returned to its home road once empty. There were financial penalties called demurrage if the car was not returned promptly. In later years, the railroad discovered ways to minimize this issue; Railbox comes mind right away.
Containers were supposed to help remedy this problem, since it was the containers themselves that were subject to imbalances. Since the containers were generally owned by other companies, the railroads successfully shifted the problem away from themselves. For a while, a serious container imbalance resulted from a large amount of traffic coming from China to the New York area. There was talk of simply scrapping the containers upon delivery of their cargo; fortunately this idea never gained much traction, but for a while, a huge number of containers were sitting empty on the east coast since nobody wanted to pay the costs of returning them.
In the case of passenger cars, it was safe to assume that in most cases that there was an equal amount of passengers in each direction, but the big variable was when they traveled. The railroads tried a variety of approaches to deal with this problem. Back in the day when college students actually used public transportation instead of private autos, the railroads would drag out every available piece of equipment when school started up or when football games were held. In many cases, these cars had been stored for months, kept on the equipment roster strictly for the brief traffic surges that happened around the beginning of school, Thanksgiving and Christmas and Spring Break. These cars were fine specimens of early 20th Century technology, with elegant and ornate light fixtures and upholstery suitable for any Victorian parlor. The witty often suggested that these cars also still had arrows sticking in their sides from Indian attacks on the prairie.
The railroads also dealt with equipment imbalances with their locomotives. As with the freight cars, locomotives often become piled up at one end of the railroad while there is a need at the other. On more than one occasion, the train enthusiast would photograph a train with 8 locomotives on the front pulling a modest train of thirty cars, far more power than the train needed. These power balancing moves were necessary to get motive power to where it was needed, and an educated ear could tell that the passing train had only two locomotives that were actually pulling.
Now, with just a few major railroads and the benefits of computerization, car and motive power imbalances are less frequent. Instead, another problem has emerged; with the downturn of freight traffic due to the difficult economy, the railroads are rushing around trying to find places to store cars which are not needed at the present.