In 1925, a plan was developed for two new tube tunnels, large enough for the Met rolling stock that would join the extension line at a junction north of Kilburn & Brondesbury station and run beneath Kilburn High Street, Maida Vale and Edgware Road to Baker Street.
I 1925 blev udviklet en plan om to nye tube-tunneler, som skulle være store nok til Mets rullende materiel, der ville forbindes til forlængelsesbanen ved en udfletning nord for Kilburn & Brondesbury Station, og køre under Kilburn High Street, Maida Vale og Edgware Road til Baker Street.