[freebeard]

Title says it all:

Just A Car Guy: The McKeen Motor Cars were the first example of an all steel, self propelled train car design in the world. They were also the first known use of purposeful aerodynamic design in North America, and the first successful use of a motive

....power other than electric or steam, and in being so, they were the lead into the Diesel/Internal Combustion engine era.