It seems to be that only the base locomotives is available to drive in the training center when you spawn in. You have to use scenario planner to be able to drive the other trains
From what I can gather it's only TSW3 specific DLC that go onto training centre. The150 is a "compatible DLC" loco so doesn't fit the criteria
Rennekton and ARuscoe are correct. The only way so far to get the Class 150 onto the training centre is to use scenario planner.