One of the biggest problems we run into in End Game is significant wait times in facilities which have had someone parked on them from before the city became a Megacity. The easiest way to solve this is to have every train hauling to a megacity park at the time the city becomes a megacity. This would allow a 1 hour cooldown for the facilities which were being hauled from, and would be much more equitable to all megacities, some of which may have more lazy people than others.
How would you fix it with industries which are used for two cities and one of the two isn't a mega city?
Players who don't want to play the endgame but want to continue normal hauling to that city?