Why was the Transcontinental railroad not built before the civil war?

It was one of the big disagreements between North and South - the North wanted the new railroad to pass through Chicago, and the South wanted it to go the New Mexico route.

To get Congress to allow the Northern route, it had to appease the South with concessions. This led to the Kansas-Nebraska Act, which allowed each new state to vote whether to be slave or free.

The first state that tried this was Kansas. Nobody realised that every bully-boy in America would now descend on thinly-populated Kansas to intimidate voters and try to invalidate the results. This was 'Bleeding Kansas', and many have identified it as a curtain-raiser for the Civil War proper.