 The 9 km Yuni - Kurioka section was doubled between 1968 and 1969, but the 4 km Kuriyama - Kurioka section became single track in 1990 as a result of the collapse of a tunnel.The 42 km Oshamambe - Toya section was double-tracked between 1964 and 1975, involving the construction of ten new tunnels and realignments that reduced the route length by 1.7 km.Several of the abandoned single-track tunnels can be seen in this section.