Hey there! I'm happy to help!
A. The railroad is considered to be America's first big business that helped with the development of other industries. One of the main things that people started working on during the Industrial Revolution was a transcontinental railroad. There was a ton of business around this, and the completion of this railroad was historic, which is why the Golden Spike in Promontory, Utah has such important historical significance.
This helped with other industries because many industries used trains to be efficient. It also helped people travel much easier and quickly, which helped other industries to develop quickly.
I hope that this helps! Have a wonderful day! :D