I've accidentally pressed "continue journey" button after doing an 66 freight service and... I'VE SPAWNED IN AN 66 ON A HST SERVICE!!!!!!! Also, 43 powercar was numbered as 00000 for some reason. I've called this train as "Night Riviera" beacuse it reminded me of it.
I was able to reproduce it, but my Xbox Series S didn't seem to like it because the fan suddenly got loud, so I turned the game off
Apparently you can trick the whole System that way. I just did a Class08 Service on GWE DL, then just for fun I clicked on "continue journey" and was then with the 08 on an HST service in the normal GWE
It seems to be an ongoing bug- sometimes "Continue journey" respawns the engine you were just driving. Happened to me too (Staring at an ICE timetable from the cab of a 155!)