I agree with churumbeque
. The railroad ties will be heavy and difficult to install--why not use the ties for the gate posts? Then you can use cheaper and easier to install posts for the rest of the paddock. Heavy posts like railroad ties should probably be set in concrete because they are so heavy.
My boss uses long t-posts (not certain at the length, but they are about 5' tall when set in the ground) and covers them with pvc pipe for safety. The electric tape that they use has connectors that attach to the pvc. I believe they use the SafeFence system. What is nice about using t-posts is that you can move them around if you need to, without breaking the bank.
At my home, we use fiberglass posts to attach our ElectroBraid fence, as well as a section of board and rail with a hot wire on the top. I must say that the ElectroBraid is my favorite fence that I have worked with--it is simple to install, safe, and economical. And most importantly, the only maintenance I've had to do is to tighten it once a year or so. ElectroBraid fence has a 30 year warranty, I believe, which is a plus. Two years back, my family and I put up a 1 acre pasture in a weekend...the hardest part is setting the corner posts in concrete. The rest is a breeze.
If you decide to use wire mesh for a fence, make sure that a horse's foot or shoe cannot get caught in it. The diamond or 'V' meshes are usually a bit safer that the larger meshes. One of the farms I used to work at had a mesh fence, and the horses didn't respect it. It would get bent, kicked, and torn up all the time. They didn't keep a hot wire on top, so perhaps that was the problem.