Between 1915 and 1917, it was further extended to Queen's Park, where it came to the surface and connected with the London and North Western Railway (LNWR), and to Watford; a total distance of 33.34 kilometres (20.72 mi).
Tahun 1915-1917, jalur kereta api kembali diperluas dan mencapai Queen's Park , di mana ia muncul ke permukaan dan terhubung dengan London dan North Western Railway (LNWR), dan ke Watford ; dengan total jarak 33,34 kilometer (20,72 mi).