There was some discussion about making DLC packages available in MPS for online use as John describes but that was quite a while ago. Now that you can export MPS routes I don't know what the direction for MPS is anymore. It seems like I haven't logged into MPS in years.
Edit: I just tried to login to a MPS route based on the West of Denver route and it offers to install it in my beta of Trainz Plus. Of course, I own that route but that is surprising.