There are two settings you can run Trainz 2009/2010 (others as well?) in and they are Native Mode and Compatibility Mode.
Compatibility Mode increases the ability of the graphics engine to handle older ways of doing things. This allows the program to run content designed for older versions of the game. This means, 2004 content is more likely to work in 2010. It is a more forgiving setting. The down side is that the graphic engine has to do more work and process more information to handle the different "types" of data. This means you lose performance.
Native Mode limits the graphic engine to the standards of the current version only. This means that the data processing is streamlined which means better performance. The down side is that often assets built for older version of the game will not work as the graphic engine will not process them.
I hope that helps! And, sorry if I did not get all the details correct!
