I just checked a recent save (Oct 1959), and in that save convoy 2301 was a 'Boeing 707-120 (high density)', serving a route between Cagbury International and Bay Airport. The image DrSuperGood posted showed that convoy 2301 was at that time and underground train. So the underground train is irrelevant to the issue - the actual bug is probably that the runway reservations weren't cleared when the plane was deleted.