The grease is applied to the wheel flange on the outer rail of the curve, to minimise wear from the flange contacting the inside of the rail, which addresses so

The grease is applied to the wheel flange on the outer rail of the curve, to minimise wear from the flange contacting the inside of the rail, which addresses some but not all screeching. Screeching can still occur because train wheels’ axles do not have a differential and one wheel travels a longer path than the other. The conical shape of the wheel profile deals with this, but some metal/metal creep between the wheels and the (ungreased top of the) rail still occurs