I have this set and have the same problem. There's another thread (which I cannot now track down) in which it's explained that the source of it is the glue originally used on the window strips in this and other Lionel passenger sets of the same period. The strips have to be replaced, which means taking the cars apart, because the adhesive chemically reacted with them and the mottled or frosted effect cannot be cleaned up. The original adhesive looks like some sort of cellulose-based contact cement.
Fortunately the replacement strips linked to above have peel and stick adhesive strips and once you get the car apart and remove the old strips they are easy to apply.
Unfortunately taking the cars apart (removing the frame and attached interior assembly) is a subject in itself. The Texas Special set is based on K-Line tooling and the frame slides into channels in sides of the body shell. Basically you have to remove the vestibule and free up the frame from the end of the car. One or both trucks may have to be removed to prevent them catching on the car's skirting but you'll notice that the trucks can be freed by taking out one screw.
So far I have only done this on the observation car of this set. I think it's probably the most difficult car to get apart and I scraped off the old adhesive to ensure it did not react with the new strips. It had set like concrete and maybe was inert but I did not want to find that this very annoying problem recurred.
Good luck with this effort! I have set aside a month of Sundays to do all of my Texas Special cars.
