Re: Third rail at the Gorge versus FRA

A third rail could be added in the Gorge with the arrival on one welded rail train containing the worn rail from a main line replacement at scrap or slightly higher price. Rail height should not be of concern as you are only talking the difference from 4" to 7.125".
Your bigger concern should be the fact that you are now into then national system where all the FRA rules now apply, including all the old ICC rules for interchange equipment. All the reporting and inspection requirements and record keeping as all interchange railroad now comply.
Unless, you establish a separate lease of the property to segregate it into a isolated right of way (private railroad) by a time period and rail removal. You can have a private isolated operation over the same R/W as a common-carrier railroad, but it has to have positive limits to prevent any overlapping operation.
Example: San Diego trolley over freight like to Mexican border. New Jersey Transit "River LINE" Trenton to Camden over Conrail freight line that is served at night. West Virginia Central from High Falls to Bergoo, WV (longest private RR in US) which is isolated by a derail for the "Salamander" to operate alone on it, removed from FRA.
Those wooden GLInc passenger cars do not meet the compression test for passenger equipment on national lines.