The Trainz serial code, on the back of the Driver's manual is always one of those tempermental things that need to be inputted exactly identical during the installation process of the game on to a PC. The serial code also has to be typed exactly from the game manual to link the user account to your game and thus can only be registered only once to prevent fraud and bootleg / second had copies of the game.
Only when the serial code has been entered properly to an user account on N3V will it then allow access to the DLS, and only when the code is typed in correctly during pre-game installation will it allow the game to install properly.
An invalid serial number could many anything not right... Game is already registered under a different user account, a second-hand copy or a bootleg disc has been obtained, or one or more of the serial code characters has been mistaken. For example 'O' has been typed in instead of ''0''. The serial code is typed in using UPPER-case characters usually to prevent a mis-type between ''O'' and '0'. Plus inbetween each four digit section is this character symbol -
The serial code is a 24 digit alphanumeric character string with the symbol - between each set of four digits, so it could even be that one of these symbols is mis-placed or missing on the user account or on the game installation process.
Andy.