I have seen - and many others will have experieinced this - regular exposure to direct sunlight behind glass start fogging the glazing, usually has a very slight blue-grey tint. Seen this on both Bachmann and Hornby's products that folks have left on windowsills and like locations. The fogging is in the bulk of the material. No cure that I know of.
[Most relevant explanation from the looks of it] Then there's chemical action. This is typically on the surface only, white or nearly so. This is a very slow form of the reactions that we will all have observed at some time where vapours from products like solvent based adhesives and cements, react on the surface of the glazing and leave a bloom. Can of petrol for the motor mower stored in the same shed where the layout is located can do it, solvents able to leak out of bottles or cans in the layout room, etc.. Storage boxes and polymer based packaging are another source, very small breakdown product of some polymer component generates a vapour which is concentrated by being enclosed in a box.
These can be fixable: polishes that restore surface smoothness and glazing materials are the agents to try. I have heard of microglaze, Krystal Klear and canopy glue among other things being successful.
Then there's 'probable fault in manufacturing'. Seen this on a couple of models. I have a single Bachmann mk 1 coach (from several dozen) which has gone opaque on a couple of windows, and have read other such reports. And a Hornby A4, on which one cab spectacle plate has gone white. Since these models were on the layout for their whole period of time in in my hands, with many other glazed models around them and none others affected, I am inclined to the view that the problem was 'built in'. No cure I know of.
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