What do you classify an entirely electrified route as then ?
I believe the original proposal here relates to freight services which run entriely under the wires, from end-to-end, not lines that are entirely electrified.No company has enough spare capacity to park up locos right left and centre at the interfaces of electrified and non electrified lines.
So, a freight service from Carlisle to Ely (using both electrified and non-electrified routes) could use diesel traction throughout without being subject to the proposed tax. Only if you have a freight service on a route such as Hitchin to Edinburgh, running on electrified lines all the way, would the operator be fined for using a diesel locomotive.
Note that my knowlege of freight flows and terminals is very limited, so I just picked on some stations that illustrate the difference.
As I said above, I don't think it is a question of which lines but rather one of which freight services. A freight service which solely uses electrified lines (and could therefore be worked using an electric locomotive) should ideally be worked by an electric locomotive. However, I don't think anybody here has seriously suggested a FOC should be penalised if they use diesel traction on electrified routes if the freight service being worked by diesel traction uses sections of non-electrified track.It's a shame that you don't answer even the basics of any question you are asked, which lines do you consider to be electrified and that diesels should not work on ?