Wooden ties are in Sweden used in main lines where there is a large risk of derailment such as in switchyards since derailed steel wheels shatter concrete ties. I have also seen them in slow speed switches and some stations.

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?