I'm of the opinion that there are two kinds of Model Railroaders, namely the ones that love to just run their trains around the track, and the ones that want to "run a railroad."
My guess is that everybody starts out as the first kind, what I'll call the "Choo-choo" railroader for lack of a better term: there is something hypnotic about the long, snaking string of cars following the parallel "iron" rails. For some people the fascination remains, while others get bored with that and "move on," either out of the hobby, or into "operations"... the "Waybill wonk" or "Timetable Tyrant."
(Let's neglect, for a moment, the Pure Craftsman—who is less interested in seeing the trains eventually run than in constructing exquisitly detailed motive power, rolling stock, structures, or scenery—since this is a fairly small minority: most folks want to see their trains run.)
I don't want to say that one is better than the other, mind you. It's just that I put myself into the latter camp; I guess I'm not as enthralled by watching a train go around in circles as I once was. For me, anyway, watching the train do circles—no matter how big the track is—becomes repetitive and monotonous... and eventually, pointless.
That's where operation seems to come to the rescue. The whole point of a railroad—at least the twelve-inch-to-the-foot variety—is to transport goods and passengers from point A to point B. Preferably in such a way as to make a profit. So this means problem-solving of the same order as used by crossword solvers... and they seem to have fun doing that. It means self-improvement (how do I solve this in fewer steps?) akin to how athletes get better at their game... and they get satisfaction from that. In this way, running a railroad becomes more than just the pride of owning a beautifully detailed model and seeing it run, but the thrill and satisfaction of a continual set of challenges overcome.
That, to me, seems like a pastime that will keep me interested for years to come.