I see

thanks for that CasyJ.. So TRS2004 and TRS2006 have no grapical changed or improvements on the engine?
Hehe good to see ur still flying
That depends.
The official statement was something like: "2004 uses some content that was made incorrectly and that may cause problems such as crashes or lower frame rates as the game engine exhausts itself working around aforementioned incorrectly made assets."
Now 2006 comes with a utility called Content Manager Plus, which will not let you install faulty assets, so you must fix them as they are installed. The assets can be fixed for 2004, and Trainz Objects is a great utility to help you do this, but 2004 will allow you to install and use "faulty" content without fixing it.
If you accept that line of reasoning, then by repairing the content when it is installed into 2004 or 2006, you may increase playability and frame rates. Some find 2006 stable on their machines, some find 2004 stable...it's a crap shoot.
Trainz Classics also uses CMP, and the bulk of the complaints about TC and 2006 stem from it, as it does take some getting used to.
The odd thing is, the built in content of 2004 was carried into 2006, and some into TC. Most of it was not repaired, but CMP was given orders to overlook the faults in built in content. TC is reported to run the best out of all the versions due to improved coding, but it hobbled by a general lack of content, the fact that the default content is either not present, or must be unlocked, and the intentional future evolution of the program means there is no "code lock". TC does have new features, such as one way roads, and headlamps on automobiles.
I have all versions of Trainz, but use 2006, only because I like to see the AI run with headlamps on all the time. I spent many hours repairing up wards of 7000 items when I transferred my content from 2004, but I feel it's been worth it.
Most content is for 2004, but will work in 2006 once tweaked. Some content requires no tweaking at all. I was assured by Rob, previously of Auran that content repaired for 2006 will transfer easily into TC and not require a great deal of tweaking. When TC reaches the end of it's incarnations I may move the content into whatever the final product is, so I feel I'm positioned well, with all of the content from 2004 in 2006 repaired and ready to move forward.
If you are not confused now, you never will be!
Doubtless someone will be along shortly to correct me, disallow my statements and assert their own, or otherwise contradict everything I have posted. That just seems to be the nature of these forums.
Doesn't matter, this thread will soon be filled with advocates of each version trumpeting their choice, as I said each person has had different experiences, so perhaps by reading them all you will be able to make a decision.
Ed