A lot of recent MTH locomotives have those rollers, not sure what the purpose it. I'm guessing the insulator on the edge is to keep the roller from dropping off the center rail but not contact switch outside rails when going through switches. However, I find that they frequently aren't as reliable as the ones without the insulation on the outside edges.
Like Dave says, clearly the insulator isn't around the whole axle. I took one out of my parts box and looked at it. If you look at the side of the roller, you can see the insulation is only a surface treatment, the metal roller is under the insulator on the axle.
