The problem is that both TRS19 Platinum and Trainz Plus when first released had the same build number - 105100 - because they have identical program code, they just differ in their permissions that give access to certain features.
Platinum is still at 105100 while Plus has been through a number of beta versions, each with increasing build numbers, since the initial release.
I still have not heard of any new betas for Platinum so the answer is still the same - no set release date for a possible Platinum update.