Indeed. I am dying to know whether the 88s, 93s ond/or 99s can work with the Mk5s. It would make speculation so much easier

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At present the only locomotives that can work with the Mk5A stock and have full functionality are the Class 68s in the number range 68019-68034.
There are two elements to the control system.
In the picture below, the two thinner wires at the top are for the ‘Wire Train Bus’ system that essentially allows the loco to be driven from the Driving Trailer (DT). This is different from the AAR system used by the Chiltern Class 68s, but similar in operation and a straightforward modification.
The second element of the system is carried via the ‘CAF’ cable which is the thicker cable at the bottom in the images. This cable carries information for the ‘Train Management System’ (TMS) and includes features such as being able to operate the coach doors from the locomotive and various safety systems (such as Hot Axel Box Detection).
In addition to the cables, 68019-68034 are fitted with additional equipment in the cabs, including coach door controls and an additional screen for the TMS.
Regarding the question of whether other classes of loco could be made compatible with the Mk5A stock, I believe the answer would be ‘yes’, but the locos would require modifications similar to those described above.
I would image that the Stadler built locos (Class 88 and Class 93) would potentially be easier, since their software and control systems share similarities with the Class 68s.
Getting the loco/stock interface working well was a big challenge with the Class 68/Mk5A sets, though it was pretty much there post Wolverton mods in 2022, so starting out with new locos might be something that would give engineers pause for thought.
Never say never, but I would discount the Class 67s. Of the classes mentioned, I think the Class 93 would be the most likely candidate for operation over electrified lines.
The Class 68s in the range 68019-34 (in reality 68019-32 since 68033/34 are owned directly by DRS) would be the only realistic candidates if the Chiltern contract did go to the Mk5A sets.