Which awnser best explains a reason people wanted to build a railroad across the United States

Because transportation across large stretches of land was relatively primitive, relying on domesticated animals, while most other systems were involving into machines, soon, transportation proved itself to be a necessity for industrialization and efficiency, and so the US pressed effort into transportation, using the new steam engine, and built a railroad stretching across the Northern half of the US (the south was less worried about industrializing, and rather profited from slave labor)