How to remedy railing swollen with rust
If it was my project, I'd get a body grinder (like those used in auto body shops), grind away much of the rust and old paint, assess the damage.   If you end up seeing that it's very badly rusted, maybe even rusted thru, it's decision time. You could patch it with a couple of steel plates epoxied, or welded to  the  lower part of the post...it would be somewhat unsightly, but when painted black, probably not too noticeable. A product like Rust-oleum would be best.   This may be a temporary solution depending upon the condition of the current post.
Also, a question:  Due to the caulking around the post, it looks like the concrete was poured after the post was installed.  Can you confirm that?   If so, replacement would be even more difficult. The existing post would need to be cut flush to the concrete and the new post would need a &quot;foot&quot; (plate) that could be screwed into the concrete.  It all depends upon the exact installation, there may be other options and hopefully others here will chime in as well.