I would close out of the Launcher and use the patch-installer instead. It does the same as is done within the Launcher except without the Launcher.
This is located in the directory (folder) where the program is installed C:\Program Files\N3V Games\TRS19, or wherever else you put it.
When you start that up, you need to specify the folder where TRS19 is located, which is the same and click on the TRS19.exe
You then need to choose the correct Trainz version you have.
In the list, ensure you are picking Platinum and not any of the beta versions.
You should then be able to then hit next, and the update should download. I'm going from memory here on the update process so I may have gotten the details a bit vauge.
Now before you start, we always recommend you backup your program its self and your data to a separate drive or location. Given the amount of data, use an external USB drive rather than thumb drives, or a separate large internal drive.