I'be petitioned in the past that widow and tractor guardian trains should be auto targeted if and only if the widow or tractor train is already in range (this includes being "in range" due to being tractored by said unit) and an auto-targeted attack order to such trains will NOT cause the ships to move to follow it (to keep attacking)*.
Unless both conditions can be implemented in the auto-targeting logic, I would rather the widow and tractor trains not be auto-targetable, as otherwise your ships would gladly throw themselves into range to just get taken by the train on their own accord. :/
Actually, given the severe effect tractor type things have on the game (alert mechanics and stuff), I would also like to have widow and tractor trains be gated at a plot intensity > 4.
*or preferably, not do so unless that train already has captured stuff, but this not being implemented should not block the idea