Creating a button/icon can be quite a pain, when it should be just the finishing move... So, here is a GIMP template that makes this as painless as possible.
You are mostly concerned with the layer called DRAW HERE. The rest of layers just helps make instantly a good icon from any uglies you put there
There is a number of layers "hidden" in a tree structure, with almost all Simutrans icon symbols known to mankind and compatible with the default skin (passengers, electricity,...). Just make visible anything you need.
Once you are done with both creative tasks, it's time to crop:
1) Delete stuff in the "stopping marks" layer, except for the mark for your tile size.
2) Flatten the folder with nearly all layers (except background).
3) Autocrop the image.
4) Delete in the resulting layer the remaining mark.
5) Flatten once more into last, single layer.
Voila... Hope you like. It helped me, certainly.