It depends on what rail your referring to. If its the Vltor cluster that Springfield is advertising...well they won't just sell the rail separately. When I inquired about it about a year or so ago (maybe longer) they said I had to send the rifle to them for the install and that the it would cost an additional $125-$150.
You can check here for M1A-related stuff too.
http://www.desertwarriorproducts.com/M1 ... tocks.html" onclick="window.open(this.href);return false;
I bought an adjustable cheekpiece from this site for my Scout which works great with the SA 3rd gen scope mount. Let me know how it works out for you if you actually get one. I've been toying with the idea of the JAE-100 stock for awhile too, but after you pimp it out it's about $1000. For that kind of money I'd rather get a McMillan for my Remmy 700 LTR. Also I did see the cluster rail at a show in Houston awhile back and it added a lot of weight to an already heavy rifle.
-Rug