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Inspecting the Bundle Of Keys from the Engineer, you see that one key unlocks access to some form of maintenance shaft beneath the ventilation chamber - except this iron corridor is massively immense; you could easily fit a dozen carriages side-by-side along its width.
You can see in this lower metal passage a large gathering of soldiers, that seem split into two camps. Though your ears are still ringing from the loud gunbattle in the vent chamber, preventing you from discerning exactly what the armed men are saying, you perceive from their gestures that the commanders of the two brigades appear to be locked in fierce argument.
