I was wondering if "skid", was the term used for the lateral motion of the tread, as it moves "sideways" across the railhead, in a curve? I don't have my books here, in the motel, but the trucks do many things, besides holding the wheels. They torsion, flex, twist and will also skew. These are extreme, in extreme curvature. The wheels don't run straight, in a curve, if they did, they would climb over the outside rail. Those wheels, while rolling, are also constantly shifting to the inside of the rail, as the wheel climbs up, off the tread, and onto the radius of the flange. I am wondering, if this constant movement, back onto the tread, is a skid? Easily seen, at very low speed, on a tight curve, it actually produces loud "popping" sounds, as it slides from the radius, back onto the tread. The other thought, was a wheel, that slowly slides, while still moving. A car moving at 20 mph, might have a dragging brake, that "skids" the wheel, at perhaps ony 8-10 mph, really a series of mini slides, and the result is built up material, on the tread, from the metal melting, being moved up, onto the tread, then cooled again. A sliding wheel, is just that. A wheel NOT rolling, and sliding along, usually burning a flat spot, with lost material being deposited on the railhead, not the wheel. Been many years, since I worked as a car knocker, and thankfully, I have forgotten most of the stuff, I learned. Probably created more questions for you, than answers, but I tried............
"Old-Timers"? I thought you WAS an old-timer, Aji...................