I’ve seen them paint ties yellow for a derail before but not anything out on the mainline like this. It’s next to these guys which I assume are just for the crossing but not sure what the white tie symbolizes.
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In my experience, it is just as Freddy says: "There's a culvert here". Because when your culvert gets clogged up, and the water is flowing OVER the tracks, and you forgot where every one of your culverts is, it's nice to see that big white marker.