Gilbert, about all this malarky of time consuming photo editing with sky backgrounds. Why not make a portable sky backscene that you can move around for each photo you take. Something flexible that can be put across the tracks, behind the buildings or anywhere. It could be made from paper or cloth, perhaps about 18 inches high and 4ft long as an experiment, a couple of upstands, one each end to support it and perhaps another in the middle for support, even stretch a piece of string between the upstands and drape an old bed sheet across it.
This thread might be of some use http://www.rmweb.co.uk/community/index.php?/topic/71294-backscene-cloths/