Do we think they’ll ever update the trains that are within the training centres with the newer dlc locos?
Doubt it. There's not enough space or memory to include every single train to be included in training center.
It's not shelved. Although they are not adding more trains to the Centre, they have moved there all the training missions for new locos. I remember unlocking all the BRO achievements without leaving the Training Centre.
Personally I would prefer the training/introduction to be done on the actual route. Nothing more annoying than unwrapping your new DLC, starting the “Journey” to find it dumps you in the TC. Especially incongruous with the steam locos.
What if there was an era appropriate version of the Training Centre, loaded with steam and diesel locos?
The most important thing about the TC is not anything you can do there, but the fact that it serves as the "depot" for all new locos and rolling stock, making them route-independent. It's why you can run the Acela on NYT without owning Boston.
I hope that if there's a package of summer routes it also includes some enhancement to the training center. It's about 25% of the way towards a really great experience at this point.