the route would continue to the then-Den-en-toshi Line and finally west, down to . As a result, the construction standards of Line 6 were based on those of Tobu and Tokyu (such as gauge track and 20-meter-long cars - today, the Mita Line is the only Toei line to use this gauge). A depot was planned at Shimura, independent of the Nishi-Magome depot on Line 1.

However, both Tokyu and Tobu decided the following year to operate their thorough services with the Teito Rapid Transit Authority