Hi Gordon
I'm still fairly new to this forum so haven't seen much of your progress in "real time". I do like what you have recently done with the lineside area near the signal box, very realistic.
I think you are close with the trees on the backscene and would definately recommend starting at the back and working forward as you seem to be doing. That way you can control how dense the final result is. As for the colours I would say you have a good blend already and don't think you need to airbrush them in this case. The trees to the left of the photo look a little flat ( but i'm not sure if these are depicting distant trees or not?) I would add more depth to the structure of them but keep it very open so you can still see through them to what you have in place now. I realise you are in the middle of the job so forgive me if you think i'm stating the obvious.
Something to give the impression of a dead tree amongst all that greenery might also bring an extra dimension to your backscene too. A suitable piece of tree root trimmed to fit could work very nicely.
I'm really enjoying watching your build and look forward to seeing more soon.
Cheers
Ant