This is just a wild suggestion and I'd take a coach you dislike from your collection to experiment on first…but,
if you've ever had to refurbish old discolored car headlights, you can sand them down with progressively finer sandpaper during which they transition from "OMG, I've completely ruined the flippin' thing!" on the first pass with rough paper, to becoming completely translucent and shining new when the finest paper is used.
If you were to be one paper rougher than 'finished', that it might solve your glare while leaving the windows sufficiently transparent to observe the interiors. I may be trial and error to find the correct finishing grit (if that's the correct term), and you'd have to remove the plastic from the coaches to do it of course. Lot of work but if they're bugging you and you're going to repaint the interiors anyway…..