How close the Cape-to-Cairo railway came to completion

Was the Cape-to-Cairo railway ever a realistic proposal?

Yes; much of it was built, and completion after the First World War was plausible before economics and decolonization changed the project.

Economics and later decolonization, rather than simple technical impossibility, prevented completion. Much of the planned route was built and remained in use, leaving a comparatively limited missing connection after the First World War. A continuous north-south line could have widened market access and made the movement of goods less dependent on fragmented transport links.