This is the lower flat of the After Engine Room.
Between the main engines number 3 and 4 is the auxiliary or "dinky" engine. This smaller engine had the advantage of being able to generate electricity even if the batteries were unavailable because the attached generator is self exciting. The main generators in contrast, require a source of field current that can come from another generator or main battery to begin generating electricity.
This engine is smaller than the main engines, but it still barely fits between them. In the photo you can see its exhaust in the center and the foundations of the main engine on either side.
All around this small space are the pipes and pumps for cooling water, lube oil and fuel oil. This space was not normally manned. The crew went into the lower flats for short, periodic checks while operating, or for repairs or cleaning.