You would need about 8000 tonnes of TNT to blow up a mountain. Consider this a mountain range, and you would need more than 12.5 million tonnes of TNT. So changing a mountain to a hill to not plausible (it is in CiV when using IGE or WorldBuilder). Maybe you can make a mod that allows worker to change mountains into a hill or instead of that, maybe Firaxis should just implement the ability to build railroads through mountains