Describe how improvements in transportation contributed to the tremendous growth of urban centers.

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.