ut controlling Congressional action, a State
may not regulate interstate commerce so as substantially to affect its
flow or deprive it of needed uniformity in its regulation is not to be
avoided by 'simply invoking the convenient apologetics of the police
power.'" So remarks Chief Justice Stone in his summarizing opinion cited
above, in Southern Pacific Co. _v._ Arizona.[818] Among others he lists
the following instances in which State legislation was invalidated on
the basis of this rule: "In the Kaw Valley case[819] the Court held that
the State was without constitutional power to order a railroad to remove
a railroad bridge over which its interstate trains passed, as a means of
preventing floods in the district and of improving its drainage, because
it was 'not pretended that local welfare needs the removal of the
defendants' bridges at the expense of the dominant requirements of
commerce with other States, but merely that it would be helped by
raising them.' And in Seaboard Air Line R. Co. _v._ Blackwell,[820] it
was held that the interference with interstate rail transportation
resulting from a State statute requiring as a safety measure that trains
come almost to a stop at grade crossings, outweigh the local interest in
safety, when it appealed that compliance increased the scheduled running
time more than six hours in a distance of one hundred and twenty-three
miles."[821] And "more recently in Kelly _v._ Washington,"[822] the
Chief Justice continued, "we have pointed out that when a State goes
beyond safety measures which are permissible because only local in their
effect upon interstate commerce, and 'attempts to impose particular
standards as to structure, design, equipment and operation [of vessels
plying interstate] which in the judgment of its authorities may be
desirable but pass beyond what is plainly essential to safety and
seaworthiness, the State will encounter the principle that such
requirements, if imposed at all, must be through the action of Congress
which can establish a uniform rule. Whether the State in a particular
matter goes too far must be left to be determined when the precise
question arises.'"

STATE REGULATION OF LENGTH OF TRAINS

Applying the test of these precedents, the Chief Justice concluded that
Arizona, in making it unlawful to operate within the State a railroad
train of more than fourteen passenger or seventy freight cars, had gone
"too far"; and in support of this conclusio