riate question for the courts" is simply whether a "commission,"
in establishing a rate, "acted within the scope of its power" and did
not violate "constitutional rights * * * by imposing confiscatory
requirements" and that a carrier, contesting the rate thus established,
accordingly was not entitled to have a court also pass upon a question
of fact regarding the reasonableness of a higher rate charged by it
prior to the order of the commission. All that need concern a court, it
said, is the fairness of the proceeding whereby the commission
determined that the existing rate was excessive; but not the expediency
or wisdom of the commission's having superseded that rate with a rate
regulation of its own.

Likewise, with a view to diminishing the number of opportunities which
courts may enjoy for invalidating rate regulations of State commissions,
the Supreme Court has placed various obstacles in the path of the
complaining litigant. Thus, not only must a person challenging a rate
assume the burden of proof,[202] but he must present a case of "manifest
constitutional invalidity";[203] and if, notwithstanding his effort, the
question of confiscation remains in doubt, no relief will be
granted.[204] Moreover, even though a public utility, which has
petitioned a commission for relief from allegedly confiscatory rates,
need not await indefinitely a decision by the latter before applying to
a court for equitable relief,[205] the latter ought not to interfere in
advance of any experience of the practical result of such rates.[206]

In the course of time, however, a distinction emerged between ordinary
factual determinations by State commissions and factual determinations
which were found to be inseparable from the legal and constitutional
issue of confiscation. In two older cases arising from proceedings begun
in lower federal courts to enjoin rates, the Court initially adopted the
position that it would not disturb such findings of fact insofar as
these were supported by substantial evidence. Thus, in San Diego Land
and Town Company _v._ National City,[207] the Court declared that: After
a legislative body has fairly and fully investigated and acted, by
fixing what it believes to be reasonable rates, the courts cannot step
in and say its action shall be set aside because the courts, upon
similar investigation, have come to a different conclusion as to the
reasonableness of the rates fixed. "Judicial interference should never
o