Original: The replacement program proceeded swiftly until 1940, when the last pre-war conversion occurred, leaving only the South London trams and the subway routes 31, 33 and 35, the only tram routes operating into North London to survive the war.
Simplified: In 1940, the last tram conversion took place before the Second World War. This left only the South London trams and subway routes 31, 33 and 35 running.