This Lionel set 6 - 29191 sold originally in 8/2006. I acquired it, in pristine condition in 12 /2018. Its been on display with occasional jaunts on my home layout.
When off the layout or shelving they reside in a really comfortable foam padded travel corrugated box stored in a very dry enviorment.
Most recently I pulled them out to service them for an upcoming open house show. To my amazement some of the windows have clouded up. It didn't happen to all the cars, just a few. I thought the clouding looked like the fogginess we've all seen on modern automotive headlamp housings. I took one car apart and applied the appropriate de-fogging cleaner with a Q-Tip.
Nothing happened! The clouding of theses lenses appears to be internal throughout the entire piece of plastic.
Please check the attached photos. If anyone has had a similar problem and was able to clear the cloudy / fogged window, I'd appreciate hearing from you.