Are you seeking to prevent weeds in the ballast or adjacent to the track (ditches, etc.)? If in and around the ties, regular movements of a locomotive with a pilot should keep things manageable for at least a while. If off to the side, a boom device with chains might work but could prove a liability (strike something and fly around) if the operator wasn't conscientious. I know that the use of herbicides is frowned upon but there are some that have a very short kill time and should not have much of an impact on the environment. Commercial companies that provide fertilizers and herbicides can be found wherever there are farms. Some might have spray equipment that could be rolled onto a flatcar at a team track.