Games like World of WarCraft take place on/in a world made 3d.

Is there software that one can use to create a world similar to say warcraft or other 3d games, you know ground, sky, buildings, trees, blah?

If so what is a good one?

Microsoft train simulator has a world creator ...but of course to get everything "right" takes an awful lot of patience Have seen some good custom scenarios Though, It does not say how many Man-hours went into producing these
Though you can alter the preloaded scenarios as well
http://en.wikipedia.org/wiki/Microsoft_Train_Simulator
Has been discounted to about $15-$20 if you shop around originally retailed @ $85! less than 3 or 4 years ago

Says on the link there is another all New version coming out based on the MS Flight Simulator engine I will look forward to that (Only a few blog postings so far) No Beta test release or alpha programs posted yet
www.microsoft.com/games/trainsimulator/
This is the only page they have

Thanks

You might be interested in Second Life. Essentially its an online world where the users create all of the game's content. You can mold the ground, erect a building, and use the built-in scripting tool to make objects perform actions. Basic accounts are free.