I have disabled all ore gen, both in the RTG and with other mods (WTF's expedition, etc.). Railcraft ores like sulfur and basalt do spawn, but not the mines.

Railcraft blog example pic: http://www.railcraft.info/wp-content/uploads/2016/10/skygen_screenshot.png

Game log:https://gist.githubusercontent.com/TitusArmillius/b455ee64dfdcea7af426aeea89de69bd/raw/09fd1c597332cb3140e0518c1999976ff5c8b991/log

In-game screenshot: http://i.imgur.com/TGLtRBL.jpg

https://github.com/CovertJaguar/Railcraft/blob/mc-1.10.2/src/main/java/mods/railcraft/common/worldgen/GeneratorMine.java#L41

Sky generation of mines seems to currently be in a permanently disabled state.