Excellent question, one that I've often pondered, and wondered about asking here as well.
I voted for "Running/operating trains" although I definitely enjoy both. If I had to choose one over the other, it would be "operation" but I still enjoy the artistic side of adding the scenery and track details.
Quite often, I think of creating the scenery as a sort of "necessary evil" needed to make the trains look better! Sometimes, I find myself rushing with the scenery so I can get to the real fun of running trains.
There's nothing like spending an evening working on the scenery, only to come downstairs in the morning (after it's dried) to admire how good it looks! :mrgreen:
Rob