Just a thought for future window plastic considerations...
I prefer thin clear polycarbonate (rather than styrene, butyrate, polyester, etc.). Midwest Products....the folks that bring you cork roadbed...sell clear polycarbonate sheet. Walthers Part # 472-70601, 2 sheets of 0.040" for about $17....or less.
Reason: Polycarbonate is more scratch-resistant than the softer, more common, less expensive plastics. Also, I like the thickness (It is also available, same source, in 0.060" and 0.080" sheets) for providing wrinkle-free appearance.
There are lots of techniques for giving a 'frosted' appearance to clear plastic sheet. Spraying one side (inside surface, as mounted in the car or building) with Dullcote or similar clear matte spray works for me.
I haven't done any tinplate cars, but I use polycarbonate for replacing the window strips on my extruded aluminum cars (Williams, Lionel, et al). I leave mine clear, since I also add seats, partitions, two-legged revenue, etc. It's held up extremely well, IMHO, of course.
FWIW, always...
KD