The first full-fledged subway line in Japan opened on 30 Dec 1927: The Tokyo Metro line between Ueno and Asakusa (approximately 2.2km). During a time in which steam trains were still the main form of locomotion, Tokyo Metro began developing the 1000 Series electric train for underground use.
To give you an idea of just how far ahead London was, here is part of an underground map from 1927: