How can I dry out and reseal this corroding railing to prevent further damage?
That's pretty far gone, basically that split is beyond economical repair.
But since you asked:
To drain it, it can be drilled from below the deck using a long drill bit, or a slot can be cut using a hacksaw with the blade sideways or reversed, or you could remove rail unit from the house and just use a regular drill.
To dry it you can heat it with a propane torch or heat gun until it stops steaming.
You can then cover the split with duct tape, might be good for a year or two, a better repair of that split seems pointless
Basically to fix this properly you need to replace that bottom rail.  That means removing the unit from the house, cutting out the old rail and welding a new one in.  After that consider having the whole thing hot dip galvanized before  re-coating it and fitting it back onto the house.  The person doing the welding probably knows a place that can do the galvanizing.