What does this line do in railcraft config anyway?
I haven't the faintest idea---I assumed either generated structures in worldgen, or perhaps something related to villages.
Edit: Yep, it's village stuff (perhaps with a Rolling Machine?) From the changelog:
NEW: Added Railcraft Villager and Structure to Villages. (credits to AEnterprise for the initial code)
Having done a bit more poking around, the Iron/Copper/Tin/Gold entries are actually kind of interesting---they generate large seams of "Poor Ore", which smelt into nuggets instead of ingots. The idea is that you now have an incentive to set up a Railcraft-based mining operation to clear out a whole deposit, which on paper sounds really fun. I don't think I'm ready to experiment with it and Gregtech and PFFA, though.