Final answer:
The railroad industry ignited the first big "boom" in the West post-Civil War, fostering the growth of related industries and becoming the nation's first major business.
Step-by-step explanation:
The first big "boom" industry of the West following the Civil War was the railroad industry. The completion of the transcontinental railroad and subsequent spur lines marked a significant increase in railway growth, underpinning the expansion of various industrial sectors. With the use of railroads, raw materials could be transported rapidly across the country, fostering other industries such as iron, wood, and coal. By the close of the century, the explosive growth of railroads with tracks spanning nearly every corner of the country reshaped the nation's economy and geography, becoming America's first "big business" leading towards major industrialization.