"The principle that, without 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.'"
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 conclusion he recites the following facts: "In Arizona, approximately 93% of the freight traffic and 95% of the passenger traffic is interstate. Because of the Train Limit Law appellant is required to haul over 30% more trains in Arizona than would otherwise have been necessary. The record shows a definite relationship between operating costs and the length of trains, the increase in length resulting in a reduction of operating costs per car. The additional cost of operation of trains complying with the Train Limit Law in Arizona amounts for the two railroads traversing that State to about $1,000,000 a year. The reduction in train lengths also impedes efficient operation. More locomotives and more manpower are required; the necessary conversion and reconversion of train lengths at terminals and the delay caused by breaking up and remaking long trains upon entering and leaving the state in order to comply with the law, delays the traffic and diminishes its volume moved in a given time, especially when traffic is heavy.