In this case the mechanism sounds believable to me. There's an ecological niche for flightless rails; going from flightworthy to flightless is presumably an eas

In this case the mechanism sounds believable to me. There's an ecological niche for flightless rails; going from flightworthy to flightless is presumably an easy step; I expect there'd be pronounced morphological changes associated with adaptation to flightlessness. If the progenitor hasn't changed much, the landlubber adaptations could make the two parallel descendants much more similar to each other than to their parents. Now, whether this is what actually happened I can't say. I have no expertise in the field.