This electrified line was built to the Japanese standard narrow gauge of 1,067 mm (3 ft 6 in) in an attempt to seek a government subsidy, and so trains from each railway could not operate on the other's tracks.

In 1963, the original terminus at Shinjuku and the streetrunning section on what is today Japan National Route 20 towards Sasazuka Station was moved to an underground alignment.

[7][8] The Keiō Line is infamous for its level crossings, of which the 25 lying on the 7.2-kilometer (4.5 mi) section between Sasazuka and Sengawa stations are classified by the Tokyo Metropolitan Government Construction Bureau as akazu no fumikiri as they are closed to road traffic for over 40 minutes in an hour.

[10] In 2016, Keiō and the Tokyo Metropolitan Government Construction Bureau proposed that the section between Sasazuka and Chofu be grade separated and widened to quadruple-track to reduce the effects caused by the present bunching on the existing at-grade double-tracked line.

[12][13] The government has planned the grade-separating project for this section of line to be completed by 2022, but this date has been delayed due to land acquisition issues.