I find that the "rotate image" feature is as important as "crop". Getting the camera down to eye-level means I cannot always precisely compose the photograph, and they are almost always skew-wiff (sp?).
I find a small bean bag (made of a latex glove filled with lentils) allows me to get to 16mm scale head height for shots.
The other trick is to make sure you match the real sunlight direction/time with a suitable donor sky-scape. In the example image the evening sun was low from the west. The Photoshopped donor skies I tried looked totally wrong until I found one that was of an evening sky looking west. Suddenly it "looked right" because it was right.
Also make your first attempt one with a real sky-line that is a smooth profile not say shrubs posing as trees as they are difficult to blend in.
Finally your real background will be slightly defocussed due to the depth of field effect of close-up photography. This looks odd: foreground slightly defocussed, train focussed, immediate real background defocussed again and then further background (the donor sky-scape) back in focus again.
This looks wrong until you deliberately blur the donor sky-scape to look slightly further out of focus than the (real) immediate background.
Sorry if there are some experts out there who are amused at my tentative attempts at photo manipulation; but if so share your tips too.
There are several other things I have learnt by trial and error that if the forum is interested I could chat about.
Andy S at the Rylston Light
Andy S. at the Rylston Light Railway