Well, a lot of any answer is going to repeat what was said on this thread: viewtopic.php?f=29&t=53018
In essence, the TTD game family are sandbox games where you have many different possible styles of play. Cooperative, competitive, creative, historical modeling. Some like playing it, some like programming for it, some like making networks, some like making new vehicles to put into the game. And as train fanatics [sorry all] tend to be geeky types, the geekiness of the game doesn't put them off, 'cos it's already far too late for them to worry about that.
I like playing on quite high difficulty settings with inflation turned on, to see how far I can get before my transport empire collapses under its own mistakes and inefficiencies. Others seek to cover the entire map with their transport network. The game's great strength, I feel, is the sheer variety of playing styles it can accomodate. Something it has in common with Kerbal Space Program, Minecraft, and, two other free games, the Battle for Wesnoth and Dwarf Fortress. I think OpenTTD is actually more accomodating of different playing styles than any of those four (and, particularly compared to Minecraft, that's /really/ saying something). /And/ OTTD was here first.