Hi G, as far as I know there isn't much that can be done with regards to railroad ties being slippery when wet (maybe someone else does?). My best advise is to use your favorite search engine, and search on cresote. This will enlighten you as to the nature of the ties and what's in them, etc.
Now if I were to take a wild guess for eliminating how slippery they were, I'd guess that something along the lines of broken glass ground into the ties. It's a long shot but probably about the only thing that may make a difference (the ties are EXTREMELY dense). Wick