must necessarily apply.

Those principles are complex and to a large degree indeterminate. On the
problem of railway and other transport rates many volumes have been
written, and many more will yet be written before a solution is arrived
at.[643] Railways, like the Post Office, are unable to allocate the
actual working costs with any degree of precision between the various
kinds of service they perform. Like the Post Office, they have one
general set of expenses, although they have diverse sources of
revenue.[644] Even if the cost of service could in each case be
definitely ascertained, its adoption as the sole basis of the rates
would prove unsatisfactory.[645] For the most part the principles on
which the rates are actually fixed resolve themselves into a
consideration of "what the traffic will bear," that is to say, the test
by actual observation and computation, strengthened, if need be, by
actual experiment, of the rates which will yield the maximum advantage
to the railway company.

The advantage to the railway conducted under private management may be
defined to be the excess of receipts from the traffic over the
out-of-pocket expenses actually incurred in handling the traffic. To
obtain this maximum it has been found necessary to vary the charge
according to the nature of the goods. Elaborate, detailed
classifications of goods have been arranged with distinct scales of
rates for each class, devised on the basis of charging each kind of
goods with the rate likely to yield to the railway the maximum of
advantage as defined above.[646] Although somewhat crude and a little
empirical, certainly largely arbitrary, this method has been almost
universally adopted for the determination of railway charges.[647]

A characteristic feature of such charges is that account is invariably
taken of the distance over which the goods are transported. In contrast
with this, the principle of uniformity of rate irrespective of distance
has been universally adopted in regard to all postal packets other than
parcels, and to some extent for parcels. The application of the
principle to parcels rests, however, on other grounds than its
application to letters. Sir Rowland Hill himself never contemplated that
the principle was necessarily applicable to all matter which might be
sent by post.[648] The circumstances under which he made his discovery,
and the facts on which he relied, make it plain that, in the absence of
other overpowering