I loved Trainz 2006 when it first came out and it was much better than UTC and Trainz 2004, I especially loved Verns route "The Glasgow to Falkirk route, but only for the annoying "clunk, click, hiss, bang" noises from all over the map, I would have stuck with Trainz 2006 until Trainz 2010 was released.
I bought Trainz 2010, because the sound was fixed, it featured the whole East Coast Mainline route from London Kings Cross to York, it featured Speedtree, which I also love, and I had a problem with silver land in Trainz 2009, which was also fixed in Trainz 2010.
I would recommend Trainz 2010 as long as you have the money to buy it, along with a good enough computer that can handle the huge routes that are included with the package, there is a lot more content in Trainz 2010 than Trainz 2006, with a lot of updated content as well.
The HST's were a dissapointment, The HST engines themselves look great, but the coaches are Paintshed reskins, even though Auran wants to dissmiss themselves from paintshed, they still use the content, even if they just painted them up a little.
Other than that, Trainz 2010 is by far the best rail simulator on the market and has beaten Railworks in many aspects, (except the HST Set, Railworks have a better looking one).
If your into UK routes, then I would also recommend Trainz Classics UK, this is the best UK rail simulator on the market, and beats Railworks hands down.
Hope this information helps
Joe Airtime