A gravy train in the hands is worth two in the gravy train station

An email I received (why?) from Michele Bachmann on behalf of the Republican National Committee is chock full of right-wing clichés (“socialist, Big Government agenda,” “takeovers of private enterprise,” “opponents of freedom”), but check out how many of them she jams into a single sentence: “These radical Democrats are spending tens of millions of dollars from their liberal special interest-filled campaign war chest to retain power and keep the publicly funded gravy train rolling into the hands of Big Labor, limousine liberals, radical protest groups, and billionaire globalists.”

Is a gravy train something you’d actually want in your hands? It sounds either very messy or very dangerous. What is a gravy train, anyway?