Everything checked off.
It hangs on entering my user name already, in each of the possible windows where credentials can be entered.
It is worthwhile to try and bring Tz12 back to life, or can I somehow merge all the DLC (almost 500000 items !?) into the latest TRS19, since T:ANE is also a passed station?
This sounds like a network issue, or possibly a corrupted database.
You can try a repair from the command prompt if you want. If this isn't a network issue.
Navigate to the directory where you installed TS12 in a command prompt. You will need to understand dos commands. You may have to use quotes like this:
CD "Program Files (x86)\N3V Games\TS12"
Once there, CD to the Bin directory.
Type in TrainzUtil repairdatabase
This will run a database repair from the command prompt. Stuff may look like it's hung, but it is not. Your prompt will reappear when the process is completed.
Regarding content:
Your DLC, meaning stuff purchased from N3V will download automatically when you run either T:ANE or TRS19 - both are valid and neither are past like TS12 is.
You won't be able to bring in built-in assets at all because they are compressed now.
Your content you purchased from third-party sites such as Jointed Rail, for example, can be reinstalled.
Importing your DLS content from TS12 will require the assets be opened for edit, thus, the reason for the repair process. If you can get past that, then you can import your assets.
Once you start up Content Manager:
Sort by installed and not built-in.
Open assets up for edit, and use the folder - import option in T:ANE or TRS19 Content Manager to bring in your content. My recommendation is to do a 100 to 1000 assets at a time. Once imported, revert those, and open up the next batch. This process will take time so be patient. I imported 197,000 assets this way. It took me about 5-days to import it all.