Install TC3 then you take the TRS2006 .ja files rename them to put trs2006 on the front then copy them into the TC folder with the TC3 .ja fires.
Your existing routes will come over easily. The TC3 game engine is more of an evolutionary than revolutionary change over TRS2006. You can download and use TRS2004, TRS2006 content quite happily. You'll probably get better frame rates depending on your hardware, TC3 can make use of more than one core ie it runs a lot better than TRS2006 on dual and quad core machines.
One way to transfer content is to create an archive in TRS2006 and bring it into TC3.
I'm not aware of any free 30 day first class ticket, I think Auran have discontinued them for all versions.
What does it give you well have a look here:
http://www.trainzclassics.co.uk/
You asked for good UK content, in my opinion TC3 has some of the best UK content available today in Trainz bar none including payware.
Also on the DLS there are some 100+ rolling stock items for TC3 most are UK based.
Paul Hobbs created some very nice wagons with animated opening side doors and many of the TC3 content on the DLS uses the same technique. I note that some one mentioned my TRS2004 wagons, OK but they aren't as stable in TRS2004 or TRS2006 as the TC3 versions and if you have problems my view is I don't have TRS2004 or TRS2006 installed so if they work they work if not upgrade. There is one known problem if you install another scripted item the scripts may interact with unpredictable results.
They use six different textures so that if you put a string of wagons down the numbers don't all look the same.
If you are into LNER look for items by jontee by the way.
Normal mapping is available in TC3. What does it mean well have a look at this link. The more detailed version uses normal mapping.
http://picasaweb.google.com/lh/photo/TraCXvUVzSCb4hzdi5LIWA?feat=directlink
Can you share items created in TC3 with some one who is running TRS2006. I don't see why not, I create for TRS2004 in TRS2009 and TC3. Just put thu correct version number in and be selective in your dependecies.
Am I demanding everyone move to TC3, I don't think so, I'm still creating TRS2004 versions of rolling stock as I recognise there are some users who accept the limitations of TRS2004, including some of the reskinners I work with. Would I like them to move onto TC3, yes, simply because it simplifies the number of different version I have to create.
If you already have TRS2006, TC3 gives you everything you have today, a better game engine that uses more than one core, and access to some very nice UK content.
Cheerio John