Eh, it wouldn't be great to weaponize. You'd have way too much warning. The prompt global strike using a submarine launched ICBM mounted with a convential warhead accomplishes everything you would want from space based weapons without your targets being able to track things as easily, yet the US doesn't do it becuase of fear it would spark diplomatic problems.
There are plenty of weapon systems better than a mag rail on the moon that we don't use today already, we don't need more.
This is the "Moon is a Harsh Mistress" scenario. The people weaponizing it may not be the Earth-bound nation state, it may be the lunar residents themselves. By the time you can run a rail system like this you've got at least a small village up there, minimum.
There's something to be said for just throwing rocks at your opponent, when your rocks have the equivalent of a small nuke in yield. They're hard to divert, there's no clever electronics tricks you can play to fool them into not detonating, it's like trying to divert a multi-ton bullet. Everything in space is a weapon, and anything capable of launching "interesting" spacecraft opens the bidding at "capable of destroying cities" and moves up from there.
Not only that - by the time Earthlings notice, they can put a whole lot of mass flying towards Earth. By tuning the mass of the individual parts, teyh can even make all of them arrive at once.
Also, the trip between Earth and Moon takes a couple days because we have limited energy. A magnetic rail, provided enough power, could launch cargo as fast as the designed limits allow. When using it as a weapon, you really don't care whether your projectile will be able to enter LEO.
Compounding that, nuclear weapons are useless on the Moon. There is no atmospheric blast, only direct radiation. Earthlings would have a hard time retaliating.
There are plenty of weapon systems better than a mag rail on the moon that we don't use today already, we don't need more.