I think that "cause" is anything that can be used as a subject. Example: Engine problem caused the train to stop. Reason is related to decision making. Example: The reason why people study is that they want to become successful. Hope it helps.

“reason for” and “cause of” both mean why something is the way it is. cause is a little more direct, as in cause and effect. to reason (verb) = to think to cause (verb) = make something happen

The reason / cause for the departure is still unkown.