That makes a lot of sense, because the weight on pilot wheels is just the wheels and the weight of whatever is holding them under the front of the loco. Except where some manufacturer has put a spring on the top of the pilot truck, those wheels don't carry any other weight. I have heard other people say that they cured pilot truck derailments by putting as much tungsten putty as possible on top of the pilot truck. (Most pilot trucks already come with metal wheels.) On a car, the trucks with the metal of plastic wheels will be carrying something like 1/2 of the car's weight, which is considerably more than the weight of just a pilot truck on a locomotive. So, it is hard to compare the effectiveness of metal wheels from one situation to the other. However, I have seen plenty of freight car trucks that were so stiff the they did not flex enough to keep all 4 of their wheels on the rails, so the car weight was not on at least one wheel. Which would probably allow that wheel to derail quite easily. Whether using metal wheels adds enough weight to bring all wheels down on the rails in stiff trucks probably depends on must how stiff the trucks are.