253 is an internet novel by Geoff Ryman. It is about the interactions between 253 people on a Tube train, as explained in this quote from the foreword:

There are seven carriages on a Bakerloo Line train, each with 36 seats. A train in which every passenger has a seat will carry 252 people. With the driver, that makes 253.

A more traditional novel about the London Underground is Barbara Vine's King Solomon's Carpet (1991).

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