No one but N3V knows. We have seen versions of Trainz Plus that had it available but it has always been very buggy and it was then removed by the team. My suspicion is that they was testing it for the Xbox version that is under development at the moment. Microsoft requires games that are released on the Xbox to support DirectX12.
Personally, I think that Trainz might at some point support DX12 but it will also continue to support DX11 as well. DX12 has many documented problems running on Nvidia graphics cards, hence the reason the Xbox has an AMD GPU in it.
