When clicking on Rail Items, in the Edit panel you’ll notice an option called Top Separator. This option serves a different purpose than the Item H. Lines option in the general Rails settings. Top Separators distinguish Rail Items from the rest in the group. For example, when placing Items linking to other pages below Items linked to Sections, try adding a Top Separator to the first Item that links to other pages. When you ungroup Items with a separator, it sets the expectation that there’s something different with between the Item groups.
If you’re NOT trying to draw attention, priority, or unique separation – but just want borders between items, it’s recommended to use the Item H. Lines option in the general Rails setup panel.