"Machinery hums behind high steel walls. Up the hill, there are visible outlines of warehouses and a building with a spire. But the lamps are low where they burn at all, and your ship the only one in harbour."
"SUFFERING HEART-ACHE? A SOOTHE & COOPER tonic for the dysphoric, despondent and depressed! Equally good for catarrh. CONVENIENT, SCIENTIFIC, INFALLIBLE!"Station III is a top-secret facility owned by a company called Soothe and Cooper. It apparently needs a constant supply of dead bodies to run, specifically the bodies of those who had taken Soothe and Cooper's Heartease Tonic and were killed permanently after overdosing due to a buildup of heartmetal - exactly what it sounds like. (This is why Soothe and Cooper Long-Boxes, aka coffins, are occasionally found in London and around the Unterzee.) It's not the bodies themselves that are needed, but rather the metal that has built up inside of them, and which is smelted down to be used again in more bottles of Heartease Tonic - and more importantly, forged into weapons.
Heartmetal is a very solid, hard, supernatural metal, little fragments of which are an ingredient of the Soothe & Copper Heart-Ease Tonic. People drink it and they don't feel depressed, but the heartmetal builds up, feeds on the grief and heartache, and grows into a solid mass that is harvested out of their chests to build weapons such as the Memento Mori.
By the way, Heartmetal occupies the same legal status as Gaoler's Honey - law-abiding citizens rarely, if ever, speak of it. It's very contraband. Don't smuggle it.