essentially now useless
Tunnels may become oval under the ground due to the weight of all that earth above. The round trains might not fit through the tunnels if this is the case. Its conceivable that a train might get stuck half way down, requiring the evacuation of the train and for several elderly (and proud) ladies to undergo the indignity of being carried down the last six hundred yards of remaining tunnel and passed up onto the platform floor.
What would happen then to the train? It would probably get hacked away with terrible cutting apparatus. It would probably get cut right down to the chassis and then rolled out. But, if the tunnel is now useless, could they will just seal up the walls and entomb the train in concrete or masonry?
It could be re-discovered by future generations and turned into a museum exhibit. No other trains might exist in such good condition. Look, they still have authentic litter on that chair. “Look at all those adverts, how rustic! Wouldn’t a print of one of those look nice on the wall next to our vertical Pheasant!” - Is what a future visitor would think. Probably.
