The foregoing discussion makes plain the error and injustice in the Postmaster-General's proposal to pay the railways for carrying the mail upon the basis of returning to them the operating expenses and taxes, as ascertained by the Post Office Department, attributable to the carriage of the mails, plus six per cent. of the sum of these expenses and taxes. The discussion under heading III above demonstrates that the plan leaves out of consideration any allowance for return upon the property and would be destructive of the universally recognized rights of the railroad companies. Furthermore, such a plan is fundamentally erroneous because it involves paying the highest rates to the railroad that by reason of physical disabilities or inefficient methods is most expensively operated and the lowest rates to the railroad which, by reason of the highest efficiency, operates at the lowest cost. A railroad's superior operating efficiency is frequently due to exceptionally heavy capital expenditures to obtain low grades, two, three or four main tracks, and to improve in other respects the roadbed and tracks to the end that trains may be hauled at the lowest expense. Such a railroad needs, and is entitled to sufficient net earnings to enable it to pay a proper return upon the increased value which is due to such expenditures. But under the Postmaster-General's plan, a railroad would be penalized for all the capital expenditures made by it for the purpose of decreasing its operating cost, because the more it decreased its operating cost the more it would decrease its mail pay. The ascertainment of the cost to a railroad of conducting mail service is necessarily very largely a matter of judgment and opinion, because a large proportion of the total operating expenses are common to the freight and passenger traffic and can only be approximately apportioned. There is room for a very wide discretion in the making of such apportionments. It would not be right or proper to entrust the Post Office Department with the discretion of making such apportionment, because the Post Office Department has an obvious interest at stake, its object always being to reduce the railroad pay to a minimum.