Magnesium doesn’t really have much use currently. About the only thing you can do with it is mix it with aluminum to make magnalium, which at least has double the durability of aluminum. It’s only real use though is making a stronger than titanium turbine. It would be nice if you could use the stuff to make rails. Even if it’s higher durability didn’t make it turn into more railcraft rails than aluminum does, at least you could use it to thin out your aluminum supply and make more rails out of a given amount of aluminum.