This is an old warehouse, shed in the compound of the former Grand Explosive Store – Naval or Dutch Naval shell store.
Maps from 1891 do not show this building, but by 1901 the building is shown and the Tramway associated with it. This Tramway ran from the Dutch Magazine, round to the Royal Naval Victualing Yard in Rosia. There was a branch of this line which went through a tunnel to the Cold Meat Store (Old North Gorge Magazine).
The railway box car shown in the image below was photographed on site in 2013. In 2015 this carriage was restored and now stands by Ragged Staff Gates.
It is believed that this rail network was in use throughout WW2.