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

coal

Explanation:

Coal typically could only be found on mountains. before the railroad expansion, coals generally only available for the consumers who live near in the mountainous area with high elevation.

The railroad expansion connected a lot regions in United States, including the mountainous region with the metropolitan region. This make the distribution cost of coal become less expensive and make this natural resources become more available to the public.