... the installers were the same for Regional, Base and Platinum from (TRS19) SP2 and beyond. Your username unlocked the features.
That is correct. In my case I purchased TRS19 Platinum. I then upgraded to Trainz Plus (via the subscription) and the change occurred without a new software download. My username and password unlocked the Trainz Plus features the next time I started what I thought was TRS19 Platinum and found that it was now Trainz Plus. If I cancel the subscription (due January each year) then when it expires my copy of Trainz Plus will revert to TRS19 Platinum without any software downloads.