by Ben Aisen and Alan Deckelbaum

Mornington Crescent is a comedy game originating on the British radio show "I'm Sorry I Haven't A Clue." The names of the host (Humphrey) and the four players (Graeme, Barry, Tim, and Willie) refer to the original cast of ISIHAC, and the scorekeeper Samantha and the postcard from Mrs. Trellis are running gags from the show.

Generally the rules of the game are so obscure and complex that to the untrained eye it looks like the players are just making them up as they go along. However, in this puzzle the rules are deceptively simple and the solver's job is to deduce what the rules are.

In each of the initial rounds, the game is played by a simple rule to get from one station to the next:

• Boston: Station's name starts with the last letter of the previous station
• Stockholm: Move two stops
• New York: Cycle through the line names alphabetically
• Paris: Count the letters in the station name, and move that many stops
• Washington: Station is across the river on the same line
• Moscow: Either the first word or the last word is the same length
• Los Angeles: Move four stops

These are obliquely hinted by the names of the rule variants used in each city.

In New York, numbered lines are out of bounds, not because they have narrower trains than the lettered lines and can't handle the "wide load", but because they'd be less intuitive to fit into the alphabetical sequence (though Graeme does suggest going to Five between F and G). Likewise, in Moscow stations with one-word names are disallowed because the rules only make sense for two-word names.

In the finals, in London, each move can be made by exactly one rule from a previous round. In each of the earlier rounds' introductions, Humphrey mentions exactly one numeral, which is used as an index into the following station's name to extract the answer phrase (TENNIS COURT):

• Wimbledon → across river (Washington/4) → West Brompton
• West Brompton → two stops (Stockholm/10) → Parsons Green
• Parsons Green → next line alphabetically (NYC/7) → Wood Lane
• Wood Lane → start with last letter (Boston/6) → Euston Square
• Euston Square → count letters (Paris/3) → Ruislip
• Ruislip → next line alphabetically (NYC/7) → Golders Green
• [break for tea]
• Oval → next line alphabetically (NYC/7) → Barons Court
• Barons Court → four stops (Los Angeles/2) → South Ealing
• South Ealing → count letters (Paris/3) → Hounslow West
• Hounslow West → last letter (Boston/6) → Totteridge & Whetstone
• Totteridge & Whetstone → word same length (Moscow/8) → Mornington Crescent

Note that the move from South Ealing to Hounslow West goes "the long way", through the Heathrow Airport loop, as mentioned in the commentary, and the starting point of Wimbledon is appropriate given the solution.