Final answer:
The central idea of the speech is that the growth of the railroad system was pivotal in connecting the nation, fostering economic expansion, and representing the progress of the United States.
Step-by-step explanation:
The central idea of Speech A is that the expansion of the railroad system has significantly transformed the United States by enhancing connectivity, promoting economic growth, and symbolizing a larger vision of progress. It emphasizes the railroad's role in reducing travel time, costs, and facilitating trade, which in turn has allowed the nation to expand both physically and ideologically. The speech underscores how innovations such as the transcontinental railroad acted as a central artery connecting various parts of the country, thereby contributing to a more unified and economically thriving nation.