You can revisit your thread here: https://ogrforum.com/...tors-ac-vs-dc?page=1
Bottom line, these toy train series wound motors typically have 3 poles and that is what truly limits the "performance". Universal series wound motors run hotter on DC because of, unlike AC, the non-existent phase angle difference between voltage and current. The AC wave also provides a cooling break that is not provided by DC. You can geek out on the math behind all of this.