If you purchased TS12 on Steam, the account needs to be linked for the product to appear on your timeline. Sometimes, this works automatically but most of the time it doesn't.
You gotta contact the Helpdesk for that. I bought TS12 a few months ago and it didn't show up on my timeline. Contacted the Helpdesk, and it popped up. May take them some time, but it will go through. Hope this helped!
T:ANE and later register using MyTrainz except for the DVD copy of T:ANE and TS12 and prior use the serial numbers, to my knowledge. Dunno who it didn't register TS12 for you if you bought it 10 years ago, though.