I could not really find something that tells me what routes add extra services and locos to other routes. I know MSB and RRO Will Add to SKO. But are than any other routes that do that to other routes??
I believe these will be updates as the preserved routes and new TSW2 routes get added. DTG started doing a good job as towards the end of TSW they had brought to life a lot of older routes by adding services, this was especially true with MSB with all of the different locomotives.
If we look at TSW, the German routes have the most locomotives, as German routes and locomotives built up in TSW for example the DB155 locomotive dlc for RSN. When it released it was added to the MSB services and so on. It created much more variety of traffic. When all of the preserved collection is added and they can add the services to routes like with TSW and we should see a lot more traffic, a good example is the first Köln stream DTG did, protagonist had the preserved collection active for the dev build and the amount of different freight and passenger traffic was a lot more then we have now!