Which line from the description of the pony express best explains the idea of "rapid transit"? it was a system by means of which messages were carried… it brought the atlantic coast and the pacific slope ten days nearer to each other. the pony express marked the highest development in overland travel… it, in fact, proved the feasibility of a transcontinental road…?

I would choose the answer, "It brought the Atlantic coast and the Pacific slope ten days nearer to each other."  That line speaks of shortening the time frame for the communication of messages by means of the Pony Express riders.  That falls in the category of "rapid transit."  Today we have even more rapid means of transit, from automobiles and trains to airplanes and jets.  We also have instantaneous communication of messages coast to coast and globally by electronic means -- a different form of "rapid transit" in the communication sector.