The quest for new concepts for the production of ties is intriguing, but coming up with a new process using virgin material that can compete with the cost and durability of concrete or timber is probably all but impossible. However, it does appear as though there is quite a bit of activity pursuing the recycled plastic composite ties. Getting rid of a waste material by casting it into dense masses that can fill the endless demand in the huge market for railroad ties seems like a real win-win proposition. But I would think there would be a devil in the details. There must be hundreds of variations in combinations of used plastic, tensile reinforcement fibers, bonding materials, and processes that might lead to the most ideal result. But with such a huge potential payoff, it really seems like fertile ground for the inventors.
Natural fibers such as hemp as mentioned above by SURF-PREP might indeed be viable as tensile members cast into polyethylene ties. Square stem rosin weed (Silphium Perfoliatum) also has strong fibers in big stems. I can imagine these low cost natural fibers being wound into rope or cords of maybe 1/8" diameter and composing a quite dense matrix where they are less than 1/16" apart with the poly filling around them. These fiber bundles would run the length of the tie just stopping short of the ends. So there would be probably as much fiber as there would be plastic in each tie. The plastic would be injection molded under high pressure, so it might be possible to completely penetrate the cords and encapsulate each fiber. If the fibers were neutralized from any natural decaying chemicals that they possess, the plastic might be a perfect protection from any moisture that could threaten their longevity. Waterproofing is one of the virtues of plastic. I would think that using recycled plastic and hemp reinforcement to produce ties would result in ties that could truly be called “green,”