Why can't I open all 4 boxrs at the train halt

[neat wharf]

I managed to keep 2 out of the 3 safes. But I can only ipe 2 if tge 4 boxes. Why?

[somber void]

I wonder if the amount of unlockable boxes related to how many safe safe.
Need specialist answers, as I've never done it (all mine were successful attempts)