fatal error on startup of save, error message:

The mod Armored Train (Turret Wagons) (0.6.3) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event Armored-train::on_tick (ID 0)
Armored-train/control.lua:364: attempt to index field 'turretA' (a nil value)
stack traceback:
Armored-train/control.lua:364: in function <Armored-train/control.lua:332>

Same here

I was able to get the error to stop after removing the mod, loading the save - overwriting and re-adding the mod.
I think this was caused by a floating turret left over after a cyclops deconstruction not working well with the update.

fatal error on startup of save, error message:

The mod Armored Train (Turret Wagons) (0.6.3) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event Armored-train::on_tick (ID 0)
Armored-train/control.lua:364: attempt to index field 'turretA' (a nil value)
stack traceback:
Armored-train/control.lua:364: in function <Armored-train/control.lua:332>

I suggest fresh install. This time is last I promise. I changed too many things, so it will take a lot of time making correct migrations (Anyway they will lead to more errors as transition from 0.6.0 - 0.6.2 is not working somehow).

P.S. One more fresh install will require when I will make grid based modular turret system (Player can choose any turret - wagon combination)

I was able to get the error to stop after removing the mod, loading the save - overwriting and re-adding the mod.
I think this was caused by a floating turret left over after a cyclops deconstruction not working well with the update.

Correct!

Should be fixed in 0.6.4!