It will depend on the specs of your hardware and the operating system you are using, but I have also found that a normal install of any version of trainz allows the program to try to run at the highest framerate it can on your system. I have found that because there can be a wide variation of the framerates when running in Driver, this can cause lag at times.
This lag can be reduced by putting the following lines in the 'trainzoptions.txt' file in the root directory of your trainz installation;
-framerate=22
-framestoaverage=16
Edit the file using Notepad, not a wordprocessor.
If you have a good system, the framerate figure can be increased, but there is no point in going above 30 since your eye will not notice any improvement even if the framerate is higher than about 25. Additionally the processor and graphics card will just run hotter for no benefit.
If you are struggling to get any decent framerates, you can try turning down 'antialiasing' in the options startup screen.
Scottish