Round Up is probably out of the question considering where you are. If you were closer I'd build
you some hydraulically driven wire brushes which would work well. Consider what you've got
available, a rail mounted air compressor would drive some air motors with rotary wire or "string"
brushes. A mechanical version could be built with some 5 hp B&S or equal motors. I think you want
to stay away from "dragging" chain over the tie ends and the right of way.