Of course, the most important way in which the economy was transformed was in the fact that it became more industrial. This meant that more and more things were mass produced in factories instead of being made by hand in shops. This was a major change.
Along with this came at least two other changes.
First, the US economy became much more nationalized. In other words, the whole country became one big market instead of being split up into a lot of little regional markets. This happened because of the huge boom in railroad construction. The railroads made it possible for goods to be shipped around the country quite easily. Therefore, people in all parts of the country could sell to one another.
Second, the economy came to be dominated by large companies. As industrialization continued, the "robber barons" created huge companies that killed off competitors. This was a huge change from an economy that was dominated by small firms run by individuals.