Expanding upon your personal anecdote: Is there scientific research on this matter? (Measuring alertness/fatigue on non-assisted vs assisted driving) It could be valuable.

Personally, I think driving is nearly always a waste of my time, so I avoid it when possible. Plus, I don't think of myself as a very good driver. Reading your anecdote made me think about how I feel after a long drive vs a long train ride. I cannot put a finger on it, but fatigue from constant required adjustments when driving /might/ be a factor.

More likely: I like how I can spend my free time when riding a train vs driving a car -- which is somewhat limited to passive listening: radio/music/audiobook/podcast/etc.