The glossy area looks like oil. I only use a dry soft cloth first. If that doesn't work, I use a mild soap and water. Rubbing alcohol if that doesn't work. Test first on a small less noticeable spot, like the end of the car.

Hi
Use dishwashing liquid highly diluted with warm water (distilled water is preferrable as it leaves no residue).
Never Ever use alcool to clean the marklin cars and locos as it dilutes the nitrate paint and the tampography (fine letters etc.
I learned this by almost ruining an HO Märklin loco with alcool.
Here is a late Märklin treasury I got. The Edelweiss
Sans titre.png (1,212kb) downloaded 62 time(s).

Cheers
Jean

Thanks to all the replies.

I'll start with highly diluted dishwash liquid in distilled water (would 1 drop in 2L be considered highly diluted ?).

I'm wary of the lighter fuel, it just sounds nasty. However it states "A weak solvent that is unlikely to affect the surface of most materials"

Then again, I have isopropyl alcohol, which is also a solvent.

I'll start on a less noticeable area with more aggressive solutions if needed.
I don't want the delicate white painted lettering to disappear.

Now for a moment, I'm not quite sure whether we actually mean the same method. It's a slightly delicate topic, so I'll try to explain:

I did not really refer to actual "spit" (a nicer expression might be saliva?), although this certainly can work too. I really mean "breathing" (in the sense of using the warmth of one's breath) onto the surface to form a coat of condensation and then immediately wiping over the surface with a soft "lens tissue" or a similar soft wiping cloth.

Apart from this detail, I fully agree with Kimball's observation that it is surprising how much grime can be removed without at all affecting the paintwork or printing by applying this "natural" method.

I hope nobody is offended by the details.

Wasn't it in the good old days that your mama would spit on a handkerchief and wipe dirt of your face?
(Not sure it ever happened to me).

Even with matt or satin paint, the Märklin locos looks much too new!
The factory-fresh look doesnt last long in real life, as dirt, rain, wind, bugs, smoke, rust, oil etc... quickly degrades the freshness. For the DB orientrot, the suns UV light also took its fair share. I think the satin look is a nice touch, to at least kill off the "worst" factory glossiness of the model.