tubechallenge.com

The Rules on the Tube Challenge page state that:

The number of each train caught must be recorded in the logbook for cross referencing with London Underground records

Travelling on the Tube yesterday I was surprised by the number of different numbers carried on the trains:

Many (though not all) trains seemed to carry a service number on their destination blinds.
Most units had a unit number on the cab front and behind the driver's internal door.
Each carriage had its own number.

Which number is it that needs to be recorded in the logbook?

It's the 3 digit number on the front of the train some are on the destination blind some are in drivers window or front end door. Exact location depends on the stock.

All though this is another piece of evidence that is misleading in the guinness book my first record we missed 3 train numbers simply because it was impossible to get them.! so dont worry for say like brixton that u havent got although those are now on side of train.