Erik, When you examine how a truck goes thru curves, the trailing axle within each truck generally assumes close to a radial orientation to the curve and the axle(s) ahead of it have an angle of attack wherein the outer wheel is grinding against the flange. On a 3-axle truck due to the long wheelbase, the leading axle is at an angle of about 1.2 deg relative to radial line. So there is lateral slippage at the contact patch with the rail that takes away force that could be tangential to the rail where it is does the tractive work. Of course, that’s a simplification but the lead outer wheel always has the greatest “angle of attack” to the rail is trying to climb it.