Stress lines in the train window

This is what the train window looked like through the polarising filter of my sun shades. It looks like the stress patterns produced by polarising filters. But I have remembered that these are normally produced by putting the plastic between two polarising filters. Could it be that the polarised sky acted asns the first filter? I didn't think of that at the time so I didn't check the direction. The rainbow colours are also interesting. Why do they repeat? They look lke the thin-film interference patterns. I didn't think to rotate the filter to see what affect the polarisation direction would have.