Hi LLilrex- The rail is steel and I usually model the car as aluminium. This makes it close to the published stiffness of the car. If you do a search here you will find stuff on how to model cars in FE. You also need to learn about car preload. Depending on what you are using the cars for you will need a preloaded car. cars bought with no preload spec will be zero preload and these will develop clearance very quickly when used on a CNC machine. This will lead to backlash very quickly... Use a medium or heavy preload car to maximise stiffness and minimise wear....Peter