Researching a trip to Korea I found it difficult to use the online tools to plan trips, especially when there are connections involved. So far I have not found a tool that works as well as the sadly lobotomised Hyperdia used to work, or even as well as the combination of JapanTravel and JapanTransit that I use today does.
Thankfully the open government data law in South Korea comes to our rescue, some spelunking turned up Korail's raw timetable and fare data available for download as heavily customised Excel spreadsheets. Using Safari and Brave on a Mac there is some funkiness with character encoding for the filenames, and the identification of the file type as a zip, but it is an xlsx and does open properly in Excel.
As an aside as an English-only speaker this is the most approachable Korean railway map that I found although I think it is a little bit out of date, in particular I think Gyeongju station is now closed. I also found this a good description of the types of passenger trains in South Korea.