Improvements in transportation led to a boom in the size of American cities in the mid-1800s. These improvements led to increased urbanization because they allowed greater amounts of food to be brought to the cities and because they allowed goods from the cities to be taken away and sold elsewhere.
These improvements in transportation went hand-in-hand with things like industrialization and immigration to increase the size of American cities. Immigrants came to the cities to work in the new factories. The immigrants could be fed because the transportation networks brought food to the cities. The products they made were valuable because they could go out on the transportation networks to be sold elsewhere.
In this way, transportation improvements made urban growth possible in the US in the mid-1800s.