Broken image, friend!

Looks more like a pile of snow, honestly. I'd say give it some larger spires and maybe some cloudcover to sell the height

I'd say your best succeeded. It's a good tree, great texture on bark and leaves. Symmetry, though rare in nature, is pleasant to the eye here. My only real suggestion would be to try a variety of trees. Dead ones, scraggly ones, droopy, bent, unusual etc.

I'd go for a permutation of the last one, but with stripes instead. Most trains have painted interiors, for one thing, so the bottom half would likely be a different shade than the top. Secondly, you really only /need/ to use that kind of pixel crosshatch to create the illusion of other colors and midtones when working with a limited pallete.
 It can work in other instances, but otherwise it is often way more productive to just use standard shading techniques so long as you can work within your color limitations.

Shadingways, it might be best to work from reference. As it's shaded now, it looks like it's supposed to be domed yet the linework remains flat. Take a look at this picture of a subway with a similar design I found:

I'm assuming you intended some permutation of the BART system, which has a slight "bubbling out" at the middle. the slope is very slight though, and as such there's not much of a shadow cast by it.