A small number of users have reported that wheels or other parts of trains are missing or ghosted out. This appears to be an issue with ATI cards, and can be solved by going to the ATI Catalyst Control Centre (usually accessed via the Control Centre in Windows) and switching off the "Adaptive Anti Aliasing" option.

ATI - Adaptive Anti Aliasing

Once this setting has been changed, click Apply/OK to save the settings and then try running Rail Simulator again to see if the problem has been solved.