- The multiple time paradoxes of 1963 make all non-linear transits problematic.
- much of the underground system in the Holborn area (which technically includes Mornington Crescent) is occupied by Yeti circa 1971
- The dinosaur invasion of 1980 disrupts surface routes
- The Cyberman invasion of 1985 disrupts all non-cyclic routes.
- Mornington Crescent is destroyed during the Dalek Invasion of 2150
Hope this helps!