They are mostly used in low or medium budget spur lines and old less used raillines. This is almost a trap for the old lines since gravel + wooden ties + segmented track is more expensive to maintain then mackadam + concrete ties + fully welded track. The trend is that all lines carrying passanger traffic or heavy cargo get fully welded lines and the rest seems to slowly be abandoned. About 80% is fully welded but we dont have as heavy axle loads as in the US, we envy your rail technology. One iron ore line use 30 000 kg/axle, most new track or renovations are built for 25 000 kg/axle and all bridges etc for 30 000 kg/axle.

The nice thing with a high axle load is that more of the total weight of the train can be cargo and with a large load profile (loading gauge?) you can fit the same load on fewer  carriages lowering the capital cost and getting higher capacity per m of switchyard track etc.

Plastic ties seems like a promising idea, especially if they can use low quality recycled plastic and perhaps even recycled fiberglass from for instance chredded windmill blades or boats?

My impression is that the lifelenght of a road depends on the roadbed quality and the axle weight of the heaviest traffic. Low quality paving seems to be almost a waste of effort since doubling the material used more then doubles the life lenght.