Why old canal dimensions were not yet a wartime obstacle

Did narrow, old British canals seriously limit wartime transport capacity?

Not to the same extent as after the war; larger postwar loads exposed constraints created especially by bridges and built-up infrastructure around the canals.

The physical canal was only part of the constraint. Rail bridges, urban development, and other structures made later enlargement expensive and disruptive, particularly where rebuilding would interrupt a main railway. During the Second World War, existing narrowboat traffic could still make effective use of the inherited network.