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I see that all the time on my silver tundra and I live nowhere near a rail road track, It comes from the sand and other crap they put on the road in the winter. Like other said, a clay bar will clear that up.
A lot of road debris in the winter contains small metal particles. This can come from spiked tires, rust from other cars, damaged rims and so on.
Or as mentioned debris from rail roads.
These small particles will rust when they come in contact with salt and moisture during the winter.
You need to remove them at least every other month, so they don't start eating at through the paint on your car.
Iron X made by carpro is the wonderfull stuff you need to buy. It smells like a donkeys A**, but does a great job of removing these rust staines.
Apply to the car, and let it work for about 5-10 minutes. Wash of.
Repeat prosedure a couple of times.
Be shure to wash the car after this is done, so you don't leave any residues.
I wouldnt say you need to remove these particles every month! That'd be overkill
I clay my car once every year and the Iron X gets put on once or twice every year. You'll find that if you do it in the spring, there will be very little fallout throughout the summer and it'll start picking up over winter again.
It doesn't necessarily come from train tracks. It can be from brake dust too or any other metal particles in the air. If you think about it - lets say you're at 70mph and brake - that brake dust is going to go everywhere including your wheels and your paint.
On lighter cars its more noticeable but if you hit a dark car with Iron X and put a tissue over it, youll see the purple bleeding from the product.