I'm thinking of things like the information on formats of files, "how to become a data manager", etc. But if it's substantially different overall structure and limited content overlap, it's probably not worth trying to create and maintain a common version with conditions to show differences.
Oh I was thinking just of the manual outlining the rules of what is or isn't included and how things are drafted, which is all that's atop this thread.
The "how to participate" page... most of the verbiage can probably be reused, though it does still need to be altered to reflect rail vs. road.
I still think it's probably easier to just have a separate copy of the page though, rather than trying to make it morph itself depending on which subdomain calls it up.