How do the train wheels turn so smoothly? After assembling a train or its companion wagons, I'm always impressed at how smoothly the wheels glide over the track.

When putting them together, it's just a metal axle with two wheels on each end inserted into the slot of the wheel holder.

Is there a special coating on the axle to limit friction?  What makes it so smooth?
Physicist Richard Feynman explains how train wheels work on this Youtube video. It's not necessary to coat the wheels, applying a generous amount of coating may decrease, traction which will effect the trains performance.