Ray - I strongly suspect that the sky is going to be a DIY paint job with torn sponge applicators - stormy skies are difficult to find.
Gene, Great idea but what sort of trees - the problem is it needs to be reasonably, but not excessively, bushy, but tall enough that the lower branches will be high enough to clear the rolling stock, and being across the pond, I don't know what grows where, except in desert regions!