The problem was the craters being rendered per-tile, interfering with the contrails. Rendering craters of all tiles in one pass and contrails in another fixes that.
This is excellent because that was the last major bug that blocked a release.
The only change that comes to mind is: dynamic and static trails were rendered separately until a few weeks ago. When I fixed the contrail memory corruptions that rendered traffic unusable, I combined the rendering of both into one loop.
