N3V/Trainz changes quickly and the fact that a "latest" Platinum beta is available but not the same beta for "Plus" makes me hesitate. But, my understanding is that pware is correct. Plus is a subscription program and not a "version" of the software. Plus subscribers get more releases (updates) of of the "Platinum" software. So, Plus and Platinum, I believe, are the same core. Plus subscribers may also have features in that core that Platinum does not (betas). And Gold is Platinum/Plus, and, add to that, "availability of all Payware content." Likewise, the "regionals" are the same software. They simply include the appropriate "regional" routes.
Every Trainz user and prospective customer should know and understand this page and the chart at the bottom:
http://www.trainzportal.com/pages/trs19-faqs
I suspect most people don't make it down to the bottom of that charge but it is an important item: "Regular New Game Feature Updates" (Plus and Gold subscriptions)