There was a rumor going around about 6 months ago that Disney was looknig for bids to build new monorails, but I haven't heard anything on that in a while. Is this real or dispelled?
However, recent rehabilitation work on the existing trains makes it seem a little less likely. I think we can say with some level of assurance that unless Disney has gotten a lot better at keeping a big secret, we're not going to see new monorail trains for the 50th anniversary.
Definitely not before the 50th, but I would think they would be sometime after that. They are in rough shape, but you never know with all the cost cutting going on.