The segments for the subway were cast using a moulding machine made to a patent owned by the British Hydraulic Co. Each segments would require the rough cast edges to be machined flat, they would then be assembled them into a ring and where necessary those rings machined again for curvature, and thus, the define the path of the subway. The rings, if left parallel would be extremely stiff when assembled together and would take the path of the subway in a straight line. The above image of segments provides an indication that even after ninety years the condition of the ironwork throughout the subway is generally serviceable. [009]

Some drawings of the moulding machine designed and patented by the British Hydraulic Company taken from a book dealing with tunneling techniques. One chapter explains that the casting of iron segments for tunnel lining does not differ greatly from other foundry work and goes on to provide the mix of ingredients used for the Blackwall tunnel i.e. 10 cwt. of No. 3 pig iron (English or Scotch), 2 cwt. of hematite, 6 cwt. of scrap (machinery, chain, etc.) and 2 cwt. of scrap (heads and gates) to make 20 cwt. or 1 Imperial ton of iron for casting! [127]