In Rule 2.1 (Board and Pieces), add after “A Piece’s location may either be “Off the Board” (notated as “-”), or at a single Station.”:-

If a Piece is at a Station, that Station’s name in the GNDT should be followed by its Line Code, in brackets. A Station’s Line Code is an alphabetical list of the first letters of each Line it is situated on (except that the Central Line is notated as “T”, the London Overground as “O” and the DLR as “R”). For example: “Barbican (CHM)”.

Then add a paragraph break.