[A] The necessity for providing, out of income, for some kinds of improvements is commonly admitted. The public constantly demands greater comfort and convenience which can be supplied only by improvements in property and equipment that bring in no additional income. A present example in the mail service, itself, is the great expense which the railways are now undergoing in substituting steel mail cars for those formerly in use. The old cars, which thus become a total loss were fully up to the most advanced standards of construction when built and they could continue for a long time to serve the purposes of the service except for the public demand for stronger cars.
[B] A car-foot mile is a unit equal to moving one foot in car length (regardless of width or height) one mile. Thus to move a car sixty feet long one mile results in sixty car-foot miles; to move the same car three miles results in 180 car-foot miles, etc.
[C] There may be some duplication in this item, but to eliminate it would require an elaborate computation which, in view of the broad margin of expenses over receipts, is wholly superfluous. Whatever duplication exists must be small in comparison with this margin.
[D] This is the sum which was apportioned by the Postmaster General on the basis of train space occupied. He estimated $40,121,294.83 (Document No. 105, page 280) as the total operating expenses and taxes of the passenger train services for the month. Of this total $21,993.06 was charged directly to the mails and $3,025,129.77 directly to the other passenger train services, leaving the sum stated in the text to be apportioned on the space basis.
