What factors contributed to the westward expansion of the u. s?

A biggie: the transcontinental railroad. Because it would economically improve wherever it was, both the North and the South competed for the railroad to be in their respective territory, evidenced by the Kansas-Nebraska Act and the Gadsen Purchase and induction of California as a free state in 1860.

In addition: people could get land cheap because it was abundant and might leave to start afresh economically.

There's other stuff I can't remember as clearly, but there's a little and I hope it helps :)