Most long-distance trains in Japan were operated by locomotives until the 1950s, but by utilizing and enhancing the technology of short-distance urban MU trains, long-distance express MU-type vehicles were developed and widely introduced starting in the mid-1950s. This work resulted in the original Shinkansen development which optimized all of the EMU's efficiencies to maximize speed. It was introduced upon completion of the Tokaido Shinkansen (literally "new trunk line") in 1964. By the 1970s, locomotive traction was regarded as slow and inefficient, and its use is now mostly limited to freight trains.