This tool fits between the trucks and reseats the bearing surface where the wheel sets run. To quote the manufacturer, “Truck Tuner’s extra-sharp tool-steel cutter reshapes the bearing to the NMRA’s recommended 60-degree contour and cleans out burrs and grime. Repeat for all bearings and your car is ready to really roll. Increases train lengths that locomotives can pull by up to 20 percent! It’s for HO scale standard gauge trucks only.”
I found this improved at least 50% of my rolling stock to the extent that I now know some of my spurs are not 100% level! Even the slightest grade causes rolling stock to do just that – roll.