Bush AF trucks to keep them from wearing through the axle/side frames. Also helps them roll with less friction when properly installed.
Using a #31 drill, open up the axle opening. Insert the bushing, re-insert the axle.
Wheel / axle picture shows cut axle from the side frame. Installing bushings prevents this from happening and also prevents the axle's from wearing upwards causing the die cast and tin trucks from shorting out to the track when going over switches.