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It can be reached by climbing up scaffolding in the junkyard to reach to the top floor of the apartment. Then stand on the ledge and use Buffjet to jump to a closer building. 15 seconds of slow time will be enough to make another jump to the rooftop with the diner.
It can also be reached without chems by climbing the collapsed building section at the northwest corner of Diamond City, traveling along the western ledge of the roof and behind the front gate. Alternatively, a jet pack can be used.
The diner has several windows, doubling as the entrance, facing away from Diamond City. Next to some of the windows is several shipping crates and a power armor station. Alongside the wall facing Diamond City are some buffet counters and minor loot. The wall facing the entrance to Diamond City has a bed and a dead settler.
The hidden diner appears only in Fallout 4.