Answer:
Completion of transcontinental railroads.
Step-by-step explanation:
One of the main reasons why the 1800s saw major improvements to trade on a nationwide scale was due to the completion of the transcontinental railroads. The railroad completely transformed the way the country did business. It allowed goods and people to travel efficiently everywhere in the country. It also allowed companies to find new markets to sell their products. The railroad connected the country in an unprecedented and extremely significant way.