Wrong. The discs are read-only and only store the software for installation. If installed on an offline computer it doesn't matter whose serial number is entered the program can still be used. There is no such thing as a "registered disc", only a registered installation.
Therefore, OP can choose to download Trainz from any source or buy a cheap secondhand Trainz and register it under his own serial number.
When you go online, only then is N3V able to check your serial number, allow/deny you access to online features and/or disable your Trainz.