AHSAN,
Trainz Classics is built on the same base code as TRS2006, with improvements made to areas that were lacking in that version. Effectively, TC uses the same Auran Jet engine, yet has vastly updated code to introduce new features and improve on old favourites, while the weather remains pretty much unchanged from TRS2006.
We are using Trainz Classics as a building block, to introduce even more features, while maintaining the same easy to use functionality that the user is familiar with in all Trainz products. In this way we can continue to develop Trainz while keep the genre attractive to the corporate side of the equation.
And there remains a lot of questions still unasked
Cheers.