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

Coal

Explanation:

The natural resource that became more available to the American public as a result of railroad expansion is coal.

As a result of rail road expansion, it was discovered that coal can be used as a source of energy which was also the most cost efficient at that time.

Using coal made transport easier across the states since most coal mines were found in the west and thus became one of the major industries in the United States.