Kant conceived of them as disjoint as a matter of our capacity to know anything about noumena, but as joined at least as a matter of our capacity for free moral action. We cannot know how we ourselves are free (since our freedom transcends our phenomenal experience of space, time, and causality), but we also could not know anything about natural phenomena without presupposing the freedom of our Reason. Thus theoretical knowledge of natural phenomena presupposes the practical freedom of the noumenal self.

Not sure if Kant is where “things went off the rails.” Philosophy has never been a train track, but is more like spelunking in uncharted caves.