Some more leaked images and a mysterious tweet by TakaraTomy suggests that the Masterpiece Trainbots will be under a new sub-line called MPG - Masterpiece Gattai (which roughly translates to Combine or Union).
Since the Trainbots are a Combiner, and the first Combiner Team to be released in the Masterpiece line, it makes sense that TakaraTomy would make a big deal over it, and package them in a special way to make it obvious that you need to collect the set to complete the combiner.



It apparently also says that the official reveal will be done very soon.