So not to put salt in the wound, the important thing is to try and avoid this for the other three. These things can become a bit of a nightmare.
To get the broken stud out, I'm thinking heat is what you need. Obviously, if the bolt is seized in there enough to break the head off, it doesn't particularly want to come out, so you need to help it to get out. I had some amazing success with wax. Yes, you heard me right: WAX.
In my case, I had gravity helping me, but the technique is as follows: Heat with a torch the opposite side of where you will be introducing the wax. Let the heat move from that opposite side to the area where you are holding the wax. Once the wax melts, it will partially be drawn into the threads by heat, and partially gravity will help it. Then wiggling the bolt back and forth will often loosen it and allow you to get it out. It has truly worked wonders for me in a couple of cases where nothing else worked.
Now having said the above, your situation presents some issues in that you can't readily get to the back of it, and gravity is working against you rather than helping you.
Option A: If I were in your situation, the first thing I would try to do is weld a bolt to the broken end of the stud. Once that is on there, I would try various forms of lubrication to allow the bolt to come free. Some folks have drilled a small hole through the rail to allow a straw from penetration oil to be slipped in there. You can also try the wax method if you can get some wax to be drawn into the area. You probably need to let this soak over time if using penetrating oil (wax works right away). Then I would try to break the bolt loose. Sometimes sharp raps on a tool rather than continued force helps to knock it loose.
Option B: If option A fails, you may be forced to further drill out the remains of the bolt and go with the largest screw extractor you can. Once it is drilled out, then I would still try to get penetrating oil up into the threads and let that sit. Finally, when trying to get it out, I would heat it up cherry red, then quicky insert the screw extractor and try to work it loose.
Those are my thoughts. I am sure others may have some ideas as well.
Good luck.
Hawk