Article I, Section 8, Clause 7:
[The Congress shall have Power . . .] To establish Post Offices and post Roads; . . .
In determining the extent to which state laws may impinge upon persons or corporations whose services are used by Congress in executing its postal powers, the task of the Supreme Court has been to determine whether particular measures are consistent with the general policies indicated by Congress. Broadly speaking, the Court has approved regulations having a trivial or remote relation to the operation of the postal service, while disallowing those constituting a serious impediment to it. Thus, a state statute, which granted to one company an exclusive right to operate a telegraph business in the state, was found to be incompatible with a federal law, which, in granting to any telegraph company the right to construct its lines upon post roads, was interpreted as a prohibition of state monopolies in a field Congress was entitled to regulate in the exercise of its combined power over commerce and post roads. 1
An Illinois statute that, as construed by the state courts, required an interstate mail train to make a detour of seven miles in order to stop at a designated station, also was held to be an unconstitutional interference with the power of Congress under this clause. 2 But a Minnesota statute requiring intrastate trains to stop at county seats was found to be unobjectionable. 3
Local laws classifying postal workers with railroad employees for the purpose of determining a railroad's liability for personal injuries, 4 or subjecting a union of railway mail clerks to a general law forbidding any
labor organization to deny any person membership because of his race, color or creed, 5 have been held not to conflict with national legislation or policy in this field. Despite the interference pro tanto with the performance of a federal function, a state may arrest a postal employee charged with murder while he is engaged in carrying out his official duties, 6 but it cannot punish a person for operating a mail truck over its highways without procuring a driver's license from state authorities. 7