Don't worry, learning anything about this has been interesting since I haven't been able to find much information about it myself probably due to it being proprietary.

The JR West Kansai main line (this), Wakayama, Kinokuni, Sakai, and maybe one or two more lines have card readers in the train, one to tap when you get on, and one for when you get off. I don't know if the trains are connected to a network at all times but I don't think they would use it on that unless it's very reliable. Or maybe there's some sort of proximity link within stations? Technical information on that system has been difficult to find too.