Stations which once were served by London Underground trains, but now exist solely on the mainline. In particular, the former Metropolitan stations between Amersham and Aylesbury, and the Bakerloo stops between Harrow & Wealdstone and Watford Junction.
Semi-ghosts occupy a controversial position in the game. Unlike full Ghosts, their status has never been expressly clarified, and many rulesets simply omit them altogether. Where declared in a game, they can lead to instability (particularly in pre-2000, 5 quadrant, rulesets) and often resolve into a particularly tight DH loop. Semi-ghosts also share the unusual feature that a winning move is impossible directly from one. [Wy]