There was this synergistic interaction between coal and iron ore deposits, trains and the steam engine. Having a portable steam engine meant that you could build trains, which meant that you could transport coal and iron ore to the steel mills, which meant you could build more steam engines, which meant that you could build more trains and expand the rail network. The fact that these things followed one another wasn't a coincidence.
And then later they realize the tar that comes out of making coal coke can be used to treat railroad ties to prevent rot, letting you lay larger networks of rail.
