it ain't the whistle that pulls the train

Being loud and boastful does nothing to actually demonstrate or prove your abilities. "Ain't" is a colloquial spelling of "isn't." Have you calculated all the potential financial implications of this new venture? You keep telling me you're some wizard with numbers, but it ain't the whistle that pulls the train. You say you're way stronger than me, so come on—let's arm wrestle and settle it for good. It ain't the whistle that pulls the train! When are these politicians going to realize that it ain't the whistle that pulls the train—that they have to actually follow through on their promises to remain in office!