Day for NIght help

We shot some day for night stuff with intent to avoid showing the sky. However, we were kind of forced to in some shots so I'm trying to see how fixable it is.

Here are three screen shots with info on the problems.
www.dropbox.com/sh/tqb3fzxcbcvo3yd/AAA3H...w88LTmRjeer_MCa?dl=0

I'm not necessarily looking for anyone to walk me through each step, but rather just some advice on if I'm on the right track or if there's a better approach I'm not thinking of. I'm new to DFN and I don't do much of this kind of grading/post work.

What I though I'd be able to do was select the sky via Luma key or Color and then extend the edge to make a nice gradient fade from the sky edge to the trees/train. I found no way of expanding/fading the edge to that degree.

the rippletwins™ recommend using a colormask ...

You an create a custom gradient matte in Pixelmator or PS, from a still of the video. Place that over the video clip, pick an appropriate blend mode, that could help. The video Karsten posted is good, too. Or use a shape mask rather than a gradient.