not a bad idea.
But we are in a very similar position as the 422 option. Old stock requiring a number of modifications and other expenses such as a power car, track, shunting etc to replace only slightly older stock. Remember this won't happen now, but rather in 5 years or more and you need to introduce DOO.
What about terminating at the various termini, Grafton, Casino, Brisbane etc and even emergency mid routes, where will the loco be turned?
Would Country link be able to follow the current path of internal drivers?
The concept would be to have double ended engines, that way if a train was terminated early for whatever means, it would do so in a loop, & therefore a simple run round is needed. The 422 in many ways was very much built as a compact locomotive, which had its advantages but also disadvantages more especially limitations on the size of the PM but also with the fuel capacity, stretched to something akin to a 442 or 80 or 81 & that allows more powerfull PM, also fuel tank, but would they really need that large a fuel tank anyway.
The Brisbane XPT fuels at Grafton one engine each direction, similar thing could happen with a diesel loco.
Countrylink has drivers qualified for diesel locomotives, as they are generally hired from the diesel side or the old diesel section, with the new break up that has Countrylink part of the interurban area, then theres scope in that direction also.
If timetabling was arranged that allowed especially on the NC, something that allowed a down service to swap off a diesel there, with one fully fueled for the run to Brisbane, or Casino, with one that has come off either another down or up service that would allow for a smoother transition along the line. ATM the Grafton XPT sits there o/night, a loco hauled service could stable the carriages, have the engine refueled & ready for either the next down or up service. Incoming driver fuels the incomming diesel off train after change.