Still a very active community from what I can gather, which is massively impressive for a game that's heading towards being 30 years old.
I think they just got a lot of simple ideas nailed from the get-go. The original TT was a little bit silly in that you could cut off the AI player(s) in their tracks (literally) - by seeing their slow progression of laying a track across a section of map. Lay a length of track across its path and the AI would jus stop - not having any clue as to what to do, be it rerouting the track or digging it back up. It made it stupidly easy to 'cheat' and block any expansion idea that the AI was having.
When the Deluxe version came out, things had improved quite a bit. Even firing up OTTD at the weekend brought back many good times from uni - huddled in a back bedroom around one PC monitor in the mid-90s, trying to financially fcuk-over the AI opposition in anyway that we could think of.