I recalled a couple mentions of the Cape to Cairo railway in Christian Wolmar's Engines of War. Sadly, the results of my search were only partially fruitful. The book does describe earlier attempts on various portions of the proposed line in good detail. This includes northern, central, and southern portions of the proposed route. However, his reference to the interwar attempt is an endnote...that references that the whole story of the Cape to Cairo can be found in his other book, Blood, Iron, & Gold, from Atlantic Books in 2009. I haven't read that book yet, so I cannot give you more information.

Interesting question, though. And overall I do highly recommend Wolmar. He not only knows his stuff, but he writes in a very accessible manner.