I'm looking to travel from Morioka to Appi Kogen in Japan. The track from Morioka to Koma is operated by Iwate Galaxy Railway (IGR), while the track from Koma to Appi Kogen is part of the JR Hanawa Line.
Can I use JR rail passes to travel this route? Or do I need to pay a supplement for IGR? Specifically interested in the Japan Rail Pass, the JR East Pass, and the Seishun 18, all of which apparently have different fare rules here to boot.
The best app for this type of thing was Hyperdia but it was discontinued a few years ago.
One alternative is navitime: https://japantravel.navitime.com/en/area/jp/route/ where you can select "JR pass only" routes. Here are a few examples for next Monday, but that's probably not what you want. So unless you want to do a huge detour, you will have to pay out of pocket.
Answering my own question, it seems to depend on the pass.
As Hilmar's answer correctly states, this journey is not free with the JR Pass: while the Pass allows travel on some third-sector railways, the Iwate Galaxy Railway is not on the list.
The Seishun 18 pass appears to apply the same rules as the JR Pass, with the guidance (in Japanese) explicitly calling out that the IGR (いわて銀河鉄道線) cannot be used even for the purpose of connecting to a JR line (ＪＲ線と直通運転している路線ならびにJRバスもご利用になれません).
However, the JR East Pass explicitly mentions that it's valid on the IGR, so this trip is possible for free.