It is not allowed to attack enemy nations via rail or conquer enemy nations even without their army.
But is it allowed to go via rail into an own embattled region?
And when no, ist their a reason (not only because it is a rule) why go from sea to land with a group transport ignoring pinning rules even if the region is embattled?
(Learning the rules for a great event this month, so be patient with me😊)
You can leave Friendly territories but not enter via rail if destination is embattled (or enemy controlled). If leaving you must follow pinning rules. As to why it works this way... I don't recall. Perhaps it is easier to retreat quickly than to reinforce quickly? I can ask Larry... but I'm guessing that is rationale.