The Fukuchiyama Line, which had fewer than 100 trains operated a day at the time of the inauguration of JR, started changing to a line with a tight railroad schedule and had over 360 trains a day after the inauguration of the JR Tozai Line in 1997 (which is still less tight than the schedule of the Hanwa Line or the Yamatoji Line that are double-tracked in the same way as the Fukuchiyama Line, but it is said that this tight schedule contributed to the JR Fukuchiyama Line Train Derailment Accident).
JR発足時は、1日の運転本数が100本足らずだった福知山線は、1997年のJR東西線開業以降は1日360本を超える過密ダイヤ（近隣の同じ複線路線である阪和線や大和路線に比べれば本数はまだ少ないが、これがJR福知山線脱線事故の一因になったともいわれる）へと変化していった。