Hello,
last year I presented here on Eurobricks a method to equip the LEGO train wheel holder with ball bearings. A disadvantage of that method (see here) was that it required to cut a small part of the wheel holder, thus you had to damage the rather expensive wheel holder. Even worse, removing the ball bearings after modification left you with a more or less useless LEGO piece, thus making it very difficult to sell these LEGO pieces in the future. Furthermore, the modification slightly added to the height of the wheel holder, i.e. the resulting holder was approximately half a plate higher than without modification.
I came up with an improvement that does not require cutting the wheel holder and does not notably add to the height of the holder. It does require cutting of a 2L technic axle though, which I do not consider a big thing since these axles are really cheap:
When inserting the red axle pieces into the holder, it is important to align them exactly as shown in the picture. When the assembly is complete, it is crucial to prevent the axle from touching the two clamps in the center of the holder.
For some additional information, click on the picture above . For a comparison of the wheel holder with and without modification, see this video: