90. A very large part of the economy which has been accomplished in railroad transportation during recent years has been due to improvements in the construction of freight-cars. In this chapter we must ignore altogether the improvements of passenger-cars, since development in passenger-car construction has followed the lines of increase in weight and in the allowable luxury of travel, rather than along economical lines. Improvements have likewise been made to increase the safety of train operations, such as improvements in couplers, air-brakes, etc. But the improvements in car construction which have tended toward economy in railroad operation have been practically confined to freight-cars. These improvements consist chiefly in increasing the strength of the car and its capacity, without proportionately increasing its deadweight. This reduction in the ratio of dead load to live load has been one of the most potent causes in the reduction of freight-charges. But these heavier cars have absolutely required improved couplers, which are not only more capable of handling the heavier loads but are less subject to deterioration and breakage. Another very potent means of economy in the handling of freight-trains is the application of air-brakes to freight-cars. In fact the very heavy trains now operated on some roads could not be safely handled, especially at such speeds as are used, without the adoption of all these improvements.

91. Weight Of Cars

The statistics furnished by the Interstate Commerce Commission give a very accurate idea of the capacity of the freight-cars used in the United States. The total number reported in 1910 was 2,133,531, with an aggregate capacity of 76,578,735 tons. The average capacity is thus about 36 tons, which is a very great increase over the corresponding figures of a few years ago. The report showed in the lightest class 1894 cars with an average capacity of 6 tons, nearly one-third of them being coal-cars. At the heavy end of the list was one car with a capacity of 280,000 pounds. Nearly 30% of the total number have the same capacity as the average (36 tons), and about 17% have a capacity of 50 tons. The increase in the average capacity during a single year was about one ton. This statement alone is a significant commentary on the economy which has been universally demonstrated in the use of cars of large capacity.

92. Ratio Of Live Load To Dead Load

A. M. Wellington, writing in 1885, gave as the weight and capacity of a "new standard box-car" 24,800 and 50,000 pounds respectively. The corresponding figures for the "old" standard cars were 20,900 and 30,000 respectively. The "new standard cars," therefore, had a carrying capacity of little over twice their dead-weight, while the carrying capacity of the old standard was about 144% of the deadweight. Since then, cars with a carrying capacity of 60,000, 80,000 and even 100,000 pounds have become not only common, but now standard. As an average figure, we may say that the car with a carrying capacity of 100,000 pounds will weigh about 38,500 pounds. The ratio of live load to dead load has been increased from 144% to 260%. When freight-cars are used to the limit of their capacity (at least in the direction of heaviest traffic), they are frequently loaded with a live load that is about 10% greater than their rated capacity. It is thus seen that the heaviest cars can be, and frequently are, loaded with a load which is about 2.86 times the dead load. There is, therefore, no exaggeration in the subsequent calculations on tonnage ratings to consider that a train of fully loaded cars has a live load which weighs twice the dead load. In fact, such a statement is conservative, considering what may be done and sometimes is done.