With my locomotive close to being completed, I already decided and will have a few great industries that real railroads service: quarry, brick factory, concrete plant, brewery, steel mill, lumber company, a scrap yard, and a charcoal factory. 

In terms of importance, steel mill, the quarry, scrap (scraps go to the steel mill), brick plant, concrete plant, lumber company, and charcoal company.

I do have a few names for some of the industris:

Coldington Steel

founded in 1872, open at current location in 1925

only been owned by the Coldington Corporation (family-owned franchise)

produce only steel products that goes into car, locomotive, rolling stock, steel rebair (separately or with for concrete rebar), "and metal yards" (steel sheets place in piles similar to a lumber yard".

Rollington Quarry

opened in 1941

limestone, gravel, and clay deposits.