Another exciting update from Ben: I have successfully 9Vified my Horizon Express.
The front headlights, however, were designed for Power Function lights (which incidentally are a much more flexible system than the old 9V train lights), which are not compatible with the 9V engine. Therefore, my HE still has an IR receiver and battery box and you need to use the remote if you want to turn the headlights on. (Redesigning the HE so it can use the old 9V headlights is definitely not a project that interests me.) If the venue is brightly lit, they probably won’t be needed anyway. (If it’s dark tho, they do look pretty cool.)
But the motor is definitely 9V, so it can be powered around 9V track w/o using up precious batteries. But it also means, of course, that it can no longer travel on RC track.