Which natural resource became more available to the American public as a result of railroad expansion?

It was "coal" that became more available to the American public as a result of railroad expansion, because many of the coal mines that were utilized were found in the west and midwest.

Explanation:

the doctrine of nullification recommended that states living inside the union have the one-sided, inalienable ideal to void any law made by the national government that could be esteemed illegal.

the united states was shaped based on a general agreement among its individual states. hence, it is inferred that under the hypothesis of invalidation, since the states are the establishment of the union, they have the ability to referee and disprove unlawful laws.