Hi Stefano,

I dont understand why the petrol does not do the job.
If the trains run successfully after cleaning, why would a few days make a difference?
Is it some temporary oxidation in the track?
I think oxidation is as big a problem as dust.

You should also clean the wheels of all your engines and rolling stock using the same method.
I use a fine wire brush originally purchased for cleaning suede leather shoes.

So the problem with petrol is that while being a solvent, it is volatile and evaporates very quickly.
Which means that it leaves no residue and maybe you get new oxidation.
I would use an oil-based cleaner such as "3-in-one" if available near you.
Or something like sewing machine oil.

On my old tin-plate track (English similar to Märklin) I use a track rubbber which is abrasive but cleans enough to keep trains running.
I don't need to do that very often.
Both Märklin and Fleischmann sell track rubbers.

Kimball