The Class 121 is NOT supplied with London-Aylesbury but a set of Chiltern Railways textures for it is included. If you wish to use these liveries then you must purchase and install the Network SouthEast Class 121 DUM Add-on (available from STEAM) BEFORE you install London-Aylesbury. If you purchase the Class 121 DMU Add-on AFTER you have installed London-Aylesbury, you will need to uninstall London-Aylesbury, install the Class 121 and then reinstall the London-Aylesbury route.