As a result of construction of the transcontinental railroad?

The Southern states wanted the route to go via New Mexico, but the Northern states wanted it to go via Chicago.

To get the South to agree the Chicago route, Congress promised the possibility of more slave-states, by allowing new states to vote on whether to be slave or free.

The first time this happened was in Kansas, when every bully-boy from both sides of the argument descended on this thinly-populated state, to intimidate voters and cause disorder. There was much bloodshed, and this unhappy period was known as 'Bleeding Kansas'.