This exercise benefited from the kind review of the Spacetrain™ engineering team. Thanks to them!
Out of respect for them and for the sake of honesty, I must therefore warn that they do not agree with the assumption used in question 4 (constant Q).
Indeed, the height h of the train and the flow rate Q are, unsurprisingly, intrinsically linked and one cannot vary without the other. I chose not to modify the statement for lack of knowing what to do better while still keeping an interesting discussion. Any ideas?