That is my understanding as well. I have Trainz Plus (TRS19 SP4) which receives quarterly updates and the next update, paid for in my subscription, will be to Trainz Plus (TRS22).
If you are a Gold member you have access to every beta release. NOTE: Make a separate install for your beta. Beta releases break things.
So... Download and install a new copy of TSR19 and set the update stream to "Plus Beta" on it. You will then get updates to the latest beta.
KEEP A SEPARATE copy of your "main" (non-beta) install for your route building and such. Each installed game will have it's own user data (content) folder.
If you don't want to do beta releases then just keep the update stream on "Plus."