Hi Simon - each axis is different and there is no reason to use the same rail on all axis. Its convenient in terms of ordering and modeling however. Plus you can rotate cars from the z when they wear to other axis if they are the same size... As a rule use the biggest rails that meets the budget. 35mm is considerably stiffer then 25mm. Looking fwd to your work...Peter
ask the motor supplier for a torque/speed chart as well...