Why do certain train cars make scratching sounds on train tracks and other train cars don’t?

Question

Why do certain train cars make scratching sounds on train tracks and other train cars don’t?

In: Other

Answer 1

If each pair of wheels on a train are attached to a single rigid axle and the train goes around the corner, one has to pass over more train track than the other which means one of them has to slide at some point (or an important moving part of the train will break).

If each wheel is able to turn independently, there’s no need for any of them to slide under normal operation.

Answer 2

Straight track or curves? Different answer for straight track.