This topic is generating an interesting conversation.
As stated previously, there are a number of different speeds being discussed here, and whilst I think that the operational speed limits are important, they obviously require more thought due to the potential impacts on the ability to operate activities. My personal thinking is that they need to be considered on a consist by consist basis as the same locomotive can have different operational speed limits applied to it depending upon track conditions, stock being hauled, etc. Trying to set this as a one off in a WAG or ENG and get it right for every scenario is impossible in MHO.
For the time being I want to focus purely on the locomotive performance "speed limits", and hence I have started a new thread here to propose a solution, and try and move just this aspect forward.
In the meantime the discussion of operational speed limits can continue in this thread.