In Factorio space age, elevated rails can't be built over tall buildings such as big-electric-pole. I suggest to add that restriction for all the cargo drone buildings. The way to do it in prototypes is to include elevated_rail in collision_mask.layers.

Thank you for suggestion and telling me how to do it
It doesn't look that great when placed in front of Elevated rails, but collision is fixed in 1.11.1

In my hubris, I've made the moorings too tall

OK, I've tried added an extra sprite to moorings that makes the top half render above Elevated rails. Not sure if that will cause issues elsewhere, but I didn't spot anything weird so I added it in 1.11.2