The other fun use case related to trains is tons of subscribers on a moving train doing handoffs between towers. Ideally everyone would use the train's wifi, but that rarely happens because the service is shit. Instead you likely have gigantic traffic hitting the MME trying to detach and reattach phones... and then everyone moves out of the new tower's service area... so on and so forth. Hopefully the 5G core's separation of some of MME's responsibilities will help this scenario.