When I was wanting new fencing to replace the 4 acred cattle fencing (40 years old) on my property, my first choice was the old, welding metal fencing. You still see some of this fencing standing up, and pushing 50yo. The expense for THAT was impossible bc nobody will weld metal affordably at that volume. The next choice was what I have seen in the last decade on Amish farms about an hour from my house. They are putting in 8" diameter fence posts and they bracket metal poles to them. Your RR ties would be just that much stronger. The metal poles are about 3-4" in diameter. I found a fencing company that put in 10' (5' high) panels of metal fencing--hard to explain, so here are some pictures~
THIS picture shows the brackets--
And that has been great for me. Above is the view from my Bedroom window. =D I know what you mean about wooden horizontal planks.
It's as if you're playing a game with your horses and they win every time you have to put a plank back or replace it. There are 2 farms close to me that use them--one is a well-healed TB broodmare farm--and I see as I drive by them about 8 sections of fence/summer that have to be repaired.
You might price heavy metal (plumbing type) poles and see if that is affordable. You could buy ALL of your materials, the metal poles and the brackets at the hardware store, and do one section at a time.
It's ALWAYS good to have a small section of electric fencing. If you buy that, too, your horses can have turnout while you work on the fencing.
One more thing...
DON'T use one of those post-hole diggers that you slam down and pull apart. You won't have arms left after putting in a fence. I have an "auger" which looks like a giant wine corkscrew and the width is adjustable. A child could dig a 3' hole with this. I just used it put in a 8' recycled fence post to secure the manger I built last November in my shelter.
This is an 8' (8" diameter) post that I dig a 3' hole to put in. Here's the manger~
Hope this helps! Let us know what you end up doing. =D