employee engaged in interstate commerce

The true test always is: Is the work in question a part of the interstate commerce in which the carrier is engaged?... Tracks and bridges are as indispensable to interstate commerce by railroad as are engines and cars; and sound economic reasons unite with settled rules of law in demanding that all of these instrumentalities be kept in repair... the work of keeping such instrumentalities in a proper state of repair while thus used is so closely related to such commerce as to be in practice and in legal contemplation a part of it.