Trainz Simulator 2009: World Builder Edition (TS2009) has everything you need to create a route, drive your favourite loco, operate multiple trains, create sessions, view your collection in Railyard and manage your content. It includes all the modules you are used to from previous versions.
In short, it is an upgrade that includes all the features we've been working on since the last full release of Trainz in 2005. It also includes the majority of past content to ensure maximum backwards compatibility with your existing routes, so it is really "Trainz 2009".
What it doesn't have is a whole range of brand new content. There are a number of new assets such as the SD40-2 loco and cab and the new TS2009 track. These assets show how good things will look once more high resolution content is produced.
To get lots more new, never released before content, including new high resolution content, you'll need to wait for Trainz Engineers Edition (TEE). TEE is a bit like Trainz Driver - it is a stand alone "Driver Only" version of Trainz aimed at people who are afraid of delving into Surveyor. Of course it will include all the cool new features of TS2009 as well.
Some people will be happy with TS2009 (WBE) and some will prefer to wait for TEE. Others will want both. The choice is yours.
Only because I have created my route favs in TRS2006 do I have apprehensions of this new thing...