I am not sure why there is a class 52 on the modern GWR line but not in action on the old Tees line or the Pennies. What do you think.
There is a simple reason for this Diesel Legenden was released after TeesValley and Northern Tranns Pennine and Layers are not retrospectively added to a Route unless Adam and his Team revise old Routes, which hasn't happened for these two Routes yet.
The 52s were a Western region locomotive. They didn't run on the Trans Pennine or the Tees Valley routes. Plus, they were all withdrawn in the 1970s so would not be around in the 1980s when the Trans Pennine and Tees Valley routes are set.
Even on a railtour they would be a stretch. You can always create a basic scenario using "Off The Rails" with the 52 for either route if you so desire.
If i want to experiment and add the 52 to the timetable for example the pennie, can i do that or would the devs have to?
The reasons why the 52s wouldn't be in the North are because the GWR was always a bit "contrary" when it came to it's motive power and the diesel hydraulics are very much part of that. If you want to experiment just do an off the rails in scenario planner and you can do what you wish
Yes: the 52s were (a) GWR/WR, not found elsewhere, and (b) mainline express passenger locos, wasted on slow, curvy secondary lines. Might as well ask why no Deltics.