There used to be a `railway=preserved` tag, however it has been deprecated.

My proposal would be to tag this line as `railway=rail` + `railway:preserved=yes` + `usage=tourism`. In my opinion, `railway=disused` is not applicable here because the line is still in use, albeit for tourism purposes. Hence why specifying `usage=tourism` would be very beneficial.

Additionally, `railway:traffic_mode=passenger` could be set, but it is not fully accurate: while this line is primarily used for passenger traffic, it can still be used for freight traffic, and freight trains can be part of heritage runs on this line.