Engine in interstate commerce

This is not like the matter of repairs upon a road permanently devoted to commerce among the states. An engine, as such, is not permanently devoted to any kind of traffic, and it does not appear that this engine was destined especially to anything more definite than such business as it might be needed for. It was not interrupted in an interstate haul to be repaired and go on. It simply had finished some interstate business and had not yet begun upon any other. ... Its character as an instrument of commerce depended on its employment at the time, not upon remote probabilities or upon accidental later events.