AEO, do you have any particular problem with Gimp? I could see if I can help with some in my companion scripts suite...
What would be helpful, is an additional 160 tile size to the simutrans tool. A proper grid that distinguishes borders between tiles and an alignment layer of the actual in-game tile, like you see on alignment templates.
I know this is one of those horrible gimp usability problems, but the layers aren't resized when selecting a tile size in the simutrans tools.
also, me updating to the latest version might be helpful
hopefully these icons are obvious in their use.
In case they are not, left to right
narrow gauge tunnel
standard gauge elevated