The swollen steel at the split is from oxidation (rust). I agree with your plan to remove trapped water in the rail. The procedure calls for moderate DIY skills and some special tools.
I'd initially start by drill a 1/4 inch hole near the split. Start drilling on the top section of the rail continuing through to and out the bottom. It may be easier to keep the drill bit on target if you strike a dimple with a punch first.
If the rail is level water may still be trap any where in the rail. But to remove from the damaged area is a good first step. So with that in mind drill again to the left and right of the split same as before.
I agree with the other person suggestion regarding applying heat to expidite evaporation of trapped water. You'll probably blister some paint but that's easily repairable. Heat gun would be best (if you have one), but a hair dryer works ,too. Propane torch as a last resort. If water is trapped you will hear and see steam eminating from your drain holes. It should evaporate under the heat source fairly quick.
I'd recommend grinding the swollen area of the split, but it isn't critical. Next you need to plug the drain holes and split. What I suggest for this is a (2) part epoxy such as JB Weld, PC-7 or an epoxy dough.
The dough is best since it won't run through the holes. Knead the two parts together until uniform in color (gray) and push into opening. It can be smoothed with acetone and a rag, but wait for it to set and then sand with a 100 grit paper.
I'd leave the bottom drain holes open so that any remaining/future moisture will not be trapped.