I haven't got time to put the rules here, but the following is a useful introduction from The Ultimate Guide to the London Underground (A48502):
"Mornington Crescent is a game for any number of players (above one). The aim is to be the first player to reach Mornington Crescent. The rules are far too numerous to list here, and really the only way to learn them is to observe at length a game played by experienced players. Basically players in turn call out the names of London Underground stations, until one player makes a mistake and gives another the opportunity to go in for the kill, by leaving them a opening to reach Mornington Crescent."
The Mornington Crescent Appreciation Society is at A769395.
If you want to find other games in H2G2 visit The Games Room at A615575.