Included in each set is bearing blocks, wheels, and axles for a single train car, or two of the example trucks shown.  Bearings are pressed as a left- and right- hand version into a 1x2 plate with pin hole.  

Obviously, plastic building bricks are not meant to have ball bearings pressed into them, and as such, the material may fatigue over time and exhibit cracking, and in rare cases, failure.  Care should be taken to avoid excessive temperature cycling, and store parts in a temperature controlled area.