I think the grey particles you see is dust, washed down by the rain.

Wipe / polish a little with a rag, and it should be gone.

In a few years, the tracks will oxidate and turn into a more brown ( and nice prototypical ) color.

Then you might need to use a little of a polishing agent like Brasso to get the top surface of the tracks clean.

https://en.wikipedia.org/wiki/Brasso

After the Brasso, you might need to wash the remains of the Brasso of.

Do NOT use sandpaper etc., since you will grind tiny lines in the surface, which will make dirt stick better to the tracks.

Cleaning is always good.

Per.

I found Peco hard work for electrical running outside. It does need cleaning to remain conducting and whilst it does weather to a really nice look for battery or steam running using track power is more problematical. Stainless steel for me is very low maintenance and that is true for Marklin although the points are prone to springs or fixings corroding as they are not Stainless. To an extent that is also true of other suppliers that the sleepers are UV resistant, the rails stainless but nuts and bolts need to be replaced either from new, which most of us are too lazy to do, or the first time you need to service something. I am in the UK so it is wet often. There is now Cliff Barker true gauge one stainless track but as my track predates it I have Aristocraft overscale G track but I can go outside for the first spring run and it does not need corrosion removing. Bird droppings or Tree sap perhaps but not natural weathering. Staying shiny is not so prototypical but neither are finger powered trains........

Graham