can you put a light rail on a 1911 that does not have one. i ame thinking about the springfield armory GI issue.

Caspian and Dawson precision both have a light rail that can be attached to a 1911s dustcover. You can screw them on but i suggest silver soldering them on.

I've installed both.  I like the Dawson since it's very unobtrusive and you can use regular holsters.  The drawback to that is that you have a dedicated light, and that it's expensive.  

The Caspian is a standard picatinny rail, and only costs $45.  You can use any light that will fit a Picatinny rail, but you will need to  find a holster, which will limit you to nylon or custom made holsters.