Which best describes the impact of railroad expansion on the steel industry?
it did not affect the demand for steel at all.
it decreased the demand for steel since railroad tracks were built from wood.
it increased the demand for steel since steel was used to make railcars and tracks.
it decreased the demand for steel since the railroad was generally not used to transport steel.

The third alternative is correct (C).

The expansion of the railways was a phenomenon of great relevance for the American economy, since it connected cities and markets, allowing the transportation of cargoes in practically the whole national territory.

In this context, all industrial sectors have benefited. In particular, the steel industry, since steel was the raw material for the train tracks, with a large increase in demand during that period.